First, you can use a Chromebook in guest mode, no Google account required (as said), but they you need to log in to your accounts every time (probably not a problem for people like my mum who log in every time they use a computer, and log out of everything when they're done.

But more to the point, lock in is when you can't move your data to another system. You need a Google account to log in (these days that's not much different from Windows or Mac basically requiring MS/Mac accounts), but you don't need to use the account for anything else you do on the Chromebook; it's just a browser. If you want to, you can export any data that browser holds (passwords, bookmarks etc) and import into any other browser. That's not lock-in. Lock-in is forcing you to continue to use a particular vendor's product because getting out of it is too difficult (usually proprietary data formats ala MS Office, or not allowing cloud exports)

How is that lock-in? Lock-in to what?

Could you not access your Google account on a Linux, macOS or Windows computer?

Could you also simply not use that Google account while using those other systems?