it's a pretty good general rule imo, but there are plenty of situations where flatting is still clearly preferable, at least exploitively.
consider if you were facing a super nitty CO opener on the button with 33 for 4% of your chips. if the CO is opening really tight your implieds for stacking him with a small pair when you spike a set are high since he'll have so many big pairs in his range. also, the chance you'll be 4-bet if you raise pre increases, which is a disaster since it denies you the good price you're getting on set mining. so in this case, calling behind with a small pocket pair is clearly better.
another example where flatting is likely preferable is if you have an opponent with a high fold to 3-bet %, and you're fairly shallow with AA or KK. (by fairly shallow i mean if 2 pot-sized bets or less would put someone all-in). here, with a hand like AA/KK and position, it's going to be easy to just shove over top of your opponents flop or turn bet and get action from wider ranges that may have just folded to a 3-bet pre.
try to think of other situations where flatting would be preferable to 3-betting. it's a good exercise to improve your thinking/game. and then also ask yourself, why is 3-betting a CO opener generally preferable vs flatting?