Thanks for the comment Cavan Huang and good question!

What's the harm in the button being potentially invisible? We don't want the user interacting with it so I see it as not a bad thing, this is why there is no need to check disabled elements with colour contrast.

I guess a better question is should we disable buttons at all?
The answer I feel would be no, but there needs to be a thorough consideration about error handling leading up to that button and if that button gets clicked with errors still present.

Which would be outside the scope of the original article.

It would be interesting to hear your thoughts.

Cheers Sean