More to the point, TFA lays out various evidence why this particular theory is unlikely.
As in tipping service? It'd be ensure anyway, surely?
The cynic would be mostly right, at least for much of the history of English, as for hundreds of years insure and ensure were simply spelling variants, and had no more difference between them than theatre and theater.
However, in the middle of the 19th century some began to find fault with this, and proposed assigning meanings to ensure, insure, and assure in more orderly fashion.
https://www.merriam-webster.com/words-at-play/how-to-use-ins...
PS: Tip is unlikely to have started as an acronym in either case.
The same. For whatever reason false etymologies based on acronyms abound, with "tip" and "posh" being among the better-known. There's also "for unlawful carnal knowledge" for fuck, "gentlemen only ladies forbidden" for golf, "constable on patrol" for cop, etc. I've even heard people claim rap derives from "rhythm and poetry". But basically they're all bunk, apart from words of quite recent coinage.
(I might add that tons of false etymologies without acronyms float around as well - "mind your Ps and Qs" being related to pints and quarts, stuff like that. Morris used to blame tour guides as being the worst culprits of keeping such things alive.)