Magnus is so incredibly strong that people think he's about to decline when suffering one (1) loss from another rising star in a niche online format. This is like when Capablanca didn't lose a single game between 1917 and 1924 and everyone acted like "the Schachmachine ist kaputt" when he broke his streak. The fact is that competition is incredibly tough and differences betwene player levels are imperceptible to all but the very most trained eyes. In fact that's why championships need so many games and tie-breakers - the people are so equal in ability that the most likely result from a match between to top level players is a draw.

Isn't he ridiculously strong at all time controls?

He is. Carlsen is a unique player in many respects and he usually comes back even stronger from losses. To say that he is "losing out" to someone is a bit premature I would say.