I just got through the second level. The claim that it's harder than Celeste doesn't seem particularly unbelievable. It was a while ago that I played Celeste, so I don't have the best memory of its difficulty, but I'd say Celeste's normal levels were easier than this game's so far.
Although it depends on when we considered the games to be cleared. Are the golden strawberries needed? Because if so, this game being more difficult does sound less probable (not that I have gotten the golden strawberries, they just sound incredibly hard, so I'm assuming they are).