I am not sure what you mean by completely true.
However, it is true enough that you have seen 3PP doing exactly what I said (on product for both - no revisions needed). It is true enough that I can use LevelUp or TotV or 2024 monsters in my 5e14 as well (and vice versa). It is true enough that I can use 2014, 2024, TotV, LevelUp classes and spells in whatever version of the game I am playing.
The idea that 3PP have to make a revision of the 2014 content to 2024 content is BS. They may chose to say that and do that for various reasons, but you don't have to.