It’s not technically true that Mac is Linux, but people say Mac “is” Linux because they are closely related and function identically for a lot of workflows.
Bear in mind that most people think of Linux in a DE-agnostic way. “Linux” isn’t what your desktop looks like, it’s a collection of a kernel and (mostly GNU) software that is largely shared between many distributions. Mac feels a lot like a different distribution of Linux, with some (or a lot of) quirks.
I’m a SWE and I work heavily in a CLI environment. I can use the same shell with the same software and the same configuration files shared between my Linux machine and my MacBook. Honestly the biggest indicator that I’m on Mac instead of Linux is that I have to remember to use homebrew instead of pacman/apt/etc. Otherwise, I was move my entire Linux workflow to Mac in a day or two, and can maintain the two environments in parallel.
Trying to do the same in windows is… frustrating, and only works at all because of the WSL letting me run a Linux pseudo-VM on top of my windows session.