Total Linux n00b here.
I was hoping to ask this question over at Zorin’s own forum but despite several attempts I’ve been unable to register. Does not bode well...
Anyway, I’m going to show off my ignorance now.
Can someone confirm that “Ubuntu compatible” necessarily means “Zorin compatible”?
It’s the hardware drivers in the kernel that count, right..?
I’m not concerned about my current hardware (which I can test with a Live CD) but for future hardware purchases I’d like some reassurance that what I buy will work with my OS of choice.

You've got the right idea about drivers being associated with the kernel and not any of the external goop around it.

As long as Zorin is using Ubuntu kernels, there's a good chance of the kernel and hardware working together, I would think.

It would probably help if you knew which Ubuntu they claim their hardware is compatible with. It looks like Zorin is using Ubuntu 17.10's 4.13 kernel, while the latest Ubuntu 18.04 LTS is running 4.15. It's possible that some new hardware would require that newer kernel, and you would either have to wait until Zorin updates or some other means of getting a more up to date kernel.

Zorin version?
https://zorinos.com/help/system-requirements/
What is your "OS of choice"?

It means its compatible with Ubuntu, the rest is...it may work it may not. That's a risk with derivatives sometimes they do things differently and it may not be compatible.

Although I don’t foresee upgrading hardware any time soon I wanted to plan ahead.
Windows PCs have always been easy to buy: I just had to filter by hardware requirements because I KNEW the OS would work just fine.
If I intend to put Zorin on my next PC then before I buy it I’ll have to google around to confirm that someone else has actually managed to do that! As Zorin’s user base is so much smaller than Ubuntu’s I was looking for some reassurance that if someone says they got Ubuntu working on their system it would be OK with Zorin too. As with most things in Linux the answer seems to be “probably” rather than “certainly”.

You'll age very quickly if you insist on looking for "certainty" or "best <whatever>" with Linux. Too many options, too many ways of doing things. Too many vendors that don't provide (decent) support.

If Zorin was 100% like Ubuntu it wouldn't have a need to exist. So the answer is "probably". And there is probably also a fix available if you find a snag.