If I screw something up in the frontend, then in most cases, I can fix it and people use the new version / reload the page and that's it. In backend, if I screw up, in most cases I now have a data problem and an ongoing problem that might only be fixable through an operational update. I might even have lost/overwritten data that is not recoverable or only through backups.
Sure, this can happen on the frontend too (e.g. sending a wrong update command to the backend API or when storing some data/configuration directly in the browser) but from my experience it is much much more likely to happened in the backend and if it does, it is more severe.
This doesn't make things more difficult on the technical side per se, but it slows things down a lot. I can't count how often I wished to be able to just truncate all user data and start with a fresh plate, instead of with a long, messed up user data history that contains quirks, fixes/workarounds in the data and that I have to stay compatible with and not forget.
So it's a stupid definition. I haven't built a large mobile app, but I'm not claiming frontend dev is easy.
You're claiming backend dev is easy but your argument so far reduces to "it's easy because the hard parts actually aren't part of it" which is tautological and pointless. You're not even just claiming it's easy, you're claiming it can be built 100% correct and bug free.
What's the most complicated backend system you've built? What database, what user scale, what traffic scale, what technologies, what servers, etc?
[0] the part of the backend that renders pages, if one exists, and the part that validates user input should be VERY VERY different components, anyway...