Exactly, so they're not the same and are not treated the same. Nobody views Gaethje as the champion, not even himself. They view him as the next challenger for the title.
And again, if someone misses weight in a undisputed title fight, it's a non title fight. Even when the champion misses weight and isn't stripped.
If someone misses weight in an interim title fight, someone can still win the "belt" because it's a glorified #1 contenders fight, they're basically guaranteed to fight for the undisputed title next. It's not a legit title because they give them out like candy to promote later title fights as "champion vs champion"