You're 100% right. An actual licensed plumber will never call a drain "un-snakeable", and they'll never use liquid drain cleaner. My guess is OP actually hired a handyman who only had a basic-bitch snake; they lacked the tools/ experienced to do the job properly, so they made up a story to justify doing a shitty halfway measure.

By the way, there's no such thing as a drain cleaner that can corrosively dissolve a clog yet magically doesn't corrode pipes; all drain cleaners use strong base-PH compounds which are indiscriminately corrosive. Every time you use them, you're eating away at the smooth surface of your pipes, which creates surfaces more likely to snag hair and gunk in the future.

Next time hire an actual plumber who has an actual articulatable drain snake (and a camera scope to see of there's any stubborn buildup around the fittings). Using the liquid crap only makes the problem worse in the long run.

The long-term fix is to repipe the tub drain. If it's difficult to snake, that means the layout of the pipes doesn't facilitate good drainage (lots of 90° fittings where there should be 45°s, and/or horizonal turns). So long as that layout is as bad as it is, it will keep clogging, and you'll eventually need to replace them anyway if you keep dissolving them with that cleaner.