I agree. But they do mean that it is a cumulonimbus from an observer's point of view. The point, which you make very eloquently, is that the observational distinction between CU and CB does not offer a definitive guide to the aviation hazards in the cloud.
I think I misunderstood what was being indicated by "wispy." If I understand correctly, you're referring to the blow-off, or anvil head on a cell, which often marks a Cb.
The blow-off, pointing to the downshear or downwind side of the cell usually is where vertical development has stopped or stymied, and often glaciated, or turned to ice crystals. it's also where a clear change in the vertical temperature lapse rate has occured, and is typically marked by windshear as well; the movement downwind of the overhang or anvil is evidence of this; it's the ice crystals being blown downwind.
This can happen with TCu, and not just a Cb cloud. While the anvil is the mark of a Cb, it doesn't necessarily mean that the cloud has become a Cb.
Your observations regarding a small cloud are well founded. I thought for a long time that if a cloud was under 15,000' tall, then it simply didn't have the energy or punch to do much damage. Unfortunately, experience has proven me wrong there. I've experienced significant damage from lightening from smaller cells than that, and experienced significant microbursts and other activity out of cells that I really didn't think had it in them. Just as importantly, I've watched cells build which began little puffy fair-weather cumulus clouds and turned into muliple raging Cb's in the time it took me to get on the ground and turn around and look up. (A few years ago I was watching weather develop on my way back from a fire. I estimated two more trips to the fire based on what I was seeing. I landed straight in, back to the weather, and turned around in the pit to start loading. I looked up through my canopy and saw a funnel cloud forming just south of the airport...and quickly tied down the airplane and went to lunch).
There's a common old wisdom with the anvil on a cell formation which says "don't fly under the anvil." I've spent a lot of time penetrating anvil and taking samples beneath and in them...and for the most part have encountered little of significance. However, they can contain ice, and despite being largely ice crystals at higher altitudes, may still contain adequate supercooled water to cause airframe icing. They may also mark the location where the hail coming out the top of a cell is going, and may have hail falling out of the anvil, too. A good policy is to avoid flying under the blowoff, or anvil. A good way to think of the anvil is as a pointer...pointing to the side of the storm where you don't want to be...also the side where turbulence and storm byproducts are most likely to be cast.
A few years ago I sat on a ramp in a small farming community, on Mother's day. I'd flown several passengers into the town in a Piaggio to spend time with their family. Forecasts had significant convective activity, but well to the east and west, with nothing of significance in our area. I noted buildups east and west, but distant, and felt comfortable with the airplane where it was. Shortly thereafter I was contacted by a crop duster on the field (we were using his ramp) to warm me about approaching weather. It was severe. It was producing severe storms, hail, tornadoes, the works.
By the time I rounded up my trusty copilot and pulled the covers off the airplane, the storm was fast approaching, and the cells were everywhere. I turned on the radar at the end of the runway and ahead of us was nothing but magenta, which is very bad...tilting up as far as I could go. I made a downwind departure with an early turnout and began working north. I moved the airplane to another location where we could get it in a hangar. As we were pulling the airplane into the hangar, tornado warnings began going off, and reports began rolling in of multiple tornados.
The storm was the biggest I'd ever seen. It largely missed us; places to the west were hit much worse. I sat at the hotel that night and watched the electrical activity in that mesocomplex, and it was continuous...continuous lightening for hours. Very impressive. What impressed me the most, however, was how rapidly it developed in an area where it wasn't forecast, and what it grew into; it was a monster. I've seen a lot of others that come up quickly and produce 70 mph winds and go away...but this one came up, produced all the damaging and dangerous stuff, and didn't go away...and was much, much bigger.
I like to think of a thunderstorm as being like the finger of God, and us in our little airplanes as being a mosquito. I think it's a good comparison as to just how small and ineffective we are in the face of such things, and it's a constant reminder to me to respect that kind of power and hazard, and to give it as wide a berth as possible.