Don't be surprised. Yes, the rules are clear. Never the less, just as the burgeoning generation of glass-and-gps trained pilots is leading to the inability to navigate between two blades of grass when the GPS is turned off, the use of equipment that is sold as having the ability to spot traffic has lead to even more lax use of the eyes in the cockpit.
"Got em' on TCAS" isn't simply a lazy response. Too often it's a report that marks the end of the effort to spot the traffic.
See and avoid applies under IFR just as VFR; the responsibility is never relieved.
TCAS is an excellent tool, with lesser versions becoming progressively less useful as the price tag and capability decreases, but it's never more than just that; a tool.
A few years ago I took a checkride in a King Air 200 with an FAA inspector. I had borrowed the airplane, which was well equipped. It included TCAS. When he climbed aboard, he saw the TCAS and groaned. We departed his home field, which was located in a heavy training area in a busy metropolitan city airspace, and immediately had a TCAS screen full of warnings, displays, and threats. Even with the threat ring reduced to a minimum, there was still a constant run of traffic alerts and warnings as we worked our way between multiple airports out to a "practice area" for air work. Even there we had more nuisance warnings than help, and the TCAS served as more of a distraction than an aid. It also called for attention often enough that the temptation to go heads down and look for it on the TCAS was a detriment to safety; the inspector turned it off.
We've used them extensively in airline service, charter, various government work, and in the fire service. I've seen them become a help, and not uncommonly, a hinderance which in some cases has compromised safety instead of enhancing it. Even in fairly hard-core visual operations, I've seen people fall victim to the tendency to let the TCAS spot traffic for them.
"Got 'em on TCAS" isn't an acceptable response to ATC. "Traffic in sight" is acceptable, as is "negative contact." But not "Got 'em on TCAS." That doesn't mean a lot to a controller, nor can the controller be assured of what you've "got" on TCAS.
"XXX traffic two o' clock and two miles, three thousand, opposite direction."
"XXX, got 'em on TCAS."
"Riiiiiiiiiight...turn left heading two seven zero, descend and maintain one thousand five hundred, report traffic in sight."