They may not have been "hacking", but they definitely are a "hacker" so it's really a matter of semantics.

It is; commonly called URL parameter tampering and it also possible to counter it:

https://www.google.com/search?q=url+parameter+(vulnerability...

That’s “hacking” in the same way as pressing a button on the elevator is trespassing.

You’re welcome to limit your definition of hacking, I prefer to keep an open mind.

Since you used an elevator as a metaphor, to me, it’s like opening an unlocked elevator control panel to reroute the signals to achieve a goal that’s not possible via a locked panel; for example, access a floor that normally would require a high security key. Easy, yes, an intended use of the system, no. Would doing so potently be trespassing? Depends on the context, but possibly, if not likely, yes.

That's an interesting example since some of the most popular DEFCON talks on youtube are about elevator hacking, which includes pressing buttons that are right there on the panel

Example of hacks that require “only” pressing buttons:

https://www.wikihow.com/Make-an-Elevator-an-Express-Elevator

That’s “hacking” in the same way as changing the page number parameter in a URL is hacking ;)