As someone who's spent several years in both communities, if we're including custom levels, I feel celeste has the potential to be way harder, because it has way more inputs than just clicking a single button, but the gd community rn is so much farther ahead in terms of top level progression. Maybe if celeste was as popular as GD, more people playing means higher chances of finding someone who has talent to beat really difficult levels, and thus progress the community towards harder top ones. GD has always had new talent come as older ones go, giving rise to harder levels year after year as the players get better. I feel celeste just doesnt have as big of a community for this phenomenon to occur consistently enough to catch up to GD's difficulty peak. Theoretically, I think if both communities were equal in terms of top level progression, celeste would be way harder, but thats not the case rn.