I have found that these things like “flat back” and “tuck your tailbone” are cues and not absolute descriptions of position. In research, it turns out that something like 90%+ people in the world have anteriorly tilted pelvises (leading to overextended backs), about 9% are posterior tilted (“tucked”), and 1% are naturally neutral. When the world is so dominantly anteriorly tilted, the most often heard cue will be “tuck your pelvis”.

Also “flat back” has been used by a ton of coaches for years – why? it’s because a lot of people tend to bend over and curl their backs due to poor posture and lack of ability to brace – or, if they are attempting certain lifts, they cannot brace properly and start dangerously rounding. But having a truly flat back, like standing against a wall and pushing your back against it until every inch of your back is touching the wall, is not optimal spine alignment. In fact, if you try it, it is actually really hard to do which hints at its unnaturalness. Thus, when the world is 90%+ has an anteriorly tilted pelvis which most often results in an overextended spine, you’ll hear someone say “flat back” because that cue, when the listener attempts it, usually brings their spines not to flat but actually to the desired neutral J curve.