I received this question a few days ago.
The answer is "I don't know". I don't know because I don't read "tips" and unsolicited advice, I don't even know where these things are shared anymore but I believe the person who asked me as they told me it is a very popular unsolicited advice. Unfortunately, the proverbial bridge between science and practice has never been built, anti-intellectualism and the anti-science attitudes are on the rise and things don't look good for literacy in general.
However, these are possible reasons:
1. The hips of whoever received this advice were actually higher than optimal for a deadlift. A deadlift is not a stiff-legged-deadlift, not an RDL, not a high snatch or clean pull, and not a squat. The triple extension of the lower limb joints to execute a deadlift should be orchestrated so that the highest number of synergistic chains participate. If the hips are high for the lift-off part of the movement (when the bar is lifted from the floor up to below the knees), there is a chance that everything will be disturbed. The deadlift becomes an SLD or an RDL but the lifter is trying to execute it as a DL. The result is the opposite of coordination: the knees extend before the hips, canceling their contribution to the overall movement, then taxing hip extension and the forces involved there, taxing lower back stabilizers which may not stabilize anything, which taxes the thoracic region, loosening the scapulae and a general screwup takes place. The lifter looks like a "C" and the best outcome is a failure. If the lifter doesn't fail, they will perform the DL with a higher chance of injury, not to mention less efficiency, power, and strength output.
2. Whoever offered unsolicited advice is, like 99.999% of social media advice suppliers, not very knowledgeable in biomechanics, physiology, and is scientifically illiterate. They are oblivious to the fact that a lift setup is strictly individual for all five official lifts and all their variations. Therefore, they ignore that the torso/femur ratio is determinant for an optimal setup and movement flow. Lifters with a higher T/F ratio will tend to start the DL with hips higher than lifters with a lower T/F ratio unless they have pretty long arms as well.
3. Because they have nothing else to do and from their list of unsolicited advice this one looked cute.
There is not a lot of knowledge in social media comments about lifting. The bullshit/qualified advice ratio is extremely high. It is so high that a low B/Q comment is too rare to be considered.
Qualified individuals may offer advice when specifically addressed and asked to provide it. Even if they chose to act in good faith and roamed the internet offering solid advice for many hours every day, their advice would be diluted in the bullshit advice ocean. I think qualified advice in this context has ceased to exist a long time ago.
