american english - Sequence of tenses when we express certainty

I beg your pardon but I have a doubt about this sentence:

"They failed to believe their eyes as they realized their world is
just an experiment"

So, if we are following a correct sequence of tense, we should use the past tense in both verbs. However, in the second clause we express certainty so the present simple ("is") appears appropriate.
Is that correct?
Thanks!