Yes, it's not that the speed of the aircraft (C150) was at the high speed buffet boundary, it could have been at 1/2 that value or even 1/3 (although my guess would put it above 1/2).
But the speed required to maintain an angle of attack that would not stall the aircraft gets faster and faster as you climb higher (the wing can't generate as much lift because the density of the air is reduced - hopefully my stating this is obvious to the ones currently in the conversation, but this is more for the OP).
So even though the speed of the aircraft is safely below the high speed buffet boundary (as we're calling it), the low speed buffet boundary (as we're calling it) gets gets faster and faster such that it reaches the maximum speed the aircraft is capable of attaining with that engine at that altitude at that temperature.
Now, since the high speed buffet is also decreasing (as far as I understand), there could also be a situation where the speed of the aircraft is faster than the high speed buffet boundary. However in something like a C150, I don't think that's very likely. I know it happens in other aircraft, because I've heard stories of Lear jets getting too high too soon (before they've lost enough weight due to fuel burn off), or of pilots trying to hand-fly a jet at high altitude in similar circumstances. Any aircraft flying faster than the high speed buffet boundary has issues, that's why it was so difficult to "break" the sound barrier. My recollection is that one test pilot discovered that the specific aircraft he was in required reversing the elevator controls at the edge of the buffet.