I think we can let this slide, but in future competitions, will not be tolerated! It does add something to the shot but I say it is ok for now only, as just a warning to people in the future, not allowed!
Well it adds something to the thumbnail, but all these images should be judged strictly on the full size version, which is why I don't think the rule really applies. I really don't see text as an "effect", I could see if the thumb was altered differently in Brightness, contrast, hue, color balance, or in another way that drastically changes the shot, which is not really the case here.
But then again, that is the rule and in the future hopefully we can catch this before a poll.
A signature... If it was a signature, it could be found on the full size image, too.

Anyways, I'd say let this slide. 👍 But next time the entry should be taken off from the poll or thelike. 👍
Unless someone thought their signature would take away from the shot, so decided not to use the signature on the full-size, but still wanted to personalize the entry in the thumbnail by providing a signature. I have faced that dilemma in the past, but I decided to use the signature on both to the entry's detriment.
^^Would the above scenario be allowed? I'm curious for future competitions.
Either way, we all need to keep an eye on the entries, and catch rule violations before the poll, so there is the opportunity to fix it. Specially with the handful of newer entrants in these competitions, I'm sure more rules will get simply overlooked in the future. If we catch it early it saves a lot of arguments and heated emotions.