Curious, would this not apply to Macs and iPads as well?

Can you access iMessage just through the Messages app on Mac?

I think for that case you need a valid mac serial number, too. I think virtualized macOS does not support iMessage either (although googling seems to suggest that you might be able to get it working by copying a valid mac serial number from another device or something. Basically, if you haven't bought a real apple device, you probably won't be able to use iMessage. "Borrowing" your friend's serial number is probably not the smartest idea either.)

Would any old MacBook, functional or not, work?

Yes, you can, but only if you are signed into your Apple ID and connected to Wi-Fi. I believe Macs and non-cellular iPads send messages over Wi-Fi to the iMessage servers.

iMessage is entirely internet-based so cellular, if present, is only used as a data connection and otherwise plays no role.

This is so weird, why specifically “Wi-Fi”? Why wouldn’t wired internet or 4G work?