三人行，必有我师(焉)

the dictionary says the top component isn't 日 rì, it's 曰 yuē ...

Huh! I was actually just taking nnozomi's word about it being a sun, but Baidu, Pleco, and Wiktionary are all telling me it is (日, not 曰). Maybe there are differing opinions about it???

(To be honest I've never actually paid that much attention to differentiating between 日/曰, since I don't think I've noticed any instances where using the wrong one would change what character you'd written. :P I should keep it in mind that they're different, though. There are a a few others radicals I always mix up, like 夂/夊 and 廴/辶...)

is it simplification or a different style of writing that radical?

I can't think of any other characters with a sun on top that are written like that in traditional, so if so, it seems like it only applies to certain specific characters, rather than a different way of writing the radical in general. But it does make sense that maybe by "simplified" they sometimes just mean "the one used on the mainland" regardless of whether that's an actual simplification or just a particular transcription style.