English is very limited when it comes to verb forms, so it tends to reuse them for different purposes.

In this case I don’t think it’s actually being used as past tense, more like a conditional, or hypothetical (not sure those are the right words, but I hope you get what I mean).

Sure, “isn’t” would probably work too, but I think using “wasn’t” might probably sound better to a native speaker (I wouldn’t know, English is my third language, but I think I’ve seen this form much more often than the present one), though it might depend on specific dialects…