To be fair, Madara's claim was proven to be shit.
But Kaguya still lives, she is sealed away. Not dead.
It wasn't quite shit, it's a simple rule that neither Ashura or Indra's feats can be performed to their
full extent by non-transmigrants. Uchihas go blind from spamming MS and Itachi died in battle as soon as he used Susanoo.
In addition, non-Uzumaki die from bijuu extraction, the Rinnegan consumed nearly all of Nagato's life force long before he used RT, Konan stated that he would die if he continued to spam Deva Path, and even Kakashi went to hospital for using his Sharingan only a couple of times.
Indra and Ashura's feats will always take a huge physical toll on a user's body if they're not
natural inheritors of their power, likewise the same would apply for Hagoromo's power.
RT is one of the Rinnegan's most advanced feats, a non-transmigrant using the jutsu is like a non-Uchiha performing Mangekyo's most advanced feats, it's only logical that Obito and Nagato would die from using it because their bodies aren't meant to harbour the Sage's power unlike Madara.