You are misreading and misinterpreting.
Have you ever actually USED the system?
Doesn’t seem like it given the fact you keep saying boneheaded things like
Proportions absolutely matter.
It’s why retargeting even exists. (along with mix matching nomenclature)
If you retarget something you get a second set of animations that are retargeted from the original.
The worse part is that I corrected you 3 times already and here you are insisting with incorrect assumptions that could possibly lead the OP and other readers to assume things that are absolutely untrue.
And what exactly makes you “sure” ?
I’m curious now, because the fact that you are sure without having ever used the system is just… magical…
I want you to take 5 minutes, grab the default character in a default template, open the run animation, and set up the re-target settings to “animation scaled” for all the bones.
That’s the same process that you would need to rely on to “ignore” (really it’s mitigate) different proportions…
Additionally, if you have an hour you should try and set up IK retargeting on a shared skeleton that has different base sizes via animation scaled - that’s a whole different problem of it’s own, but still something you should probably be aware of by doing.