When would they reject a paper in case of violating the double-blind review policy?
It very much depends on the conference and the reviewers how strict the rules are applied, but in general, it is the responsibility of both the author and the reviewer to not circumvent the double-blind policy. Thus a reviewer should not intentionally invest any effort to find out who the author of the paper is unless it is obvious from the submission itself (a common mistake is referencing own work too obviously).

Double-blind rarely means that you are not allowed to have previous work on a similar topic. There are researchers who publish every year at the same conference with double-blind submission policy. Of course, everybody knows who the author is even if it is not written anywhere in the paper, but just because their work is known in the community. So your case is slightly different because I assume that the name and content of your talk are the same of your submission, so the probability is higher to find your talk if someone searches for the title, but as written above, reviewers should not do that.

Of course, prior official publication of the same paper is usually not allowed, but that does not seem the case for you. In any case, I never came across "desk-rejection" at conferences, so you have to wait until the official notification date.