It’s worth noting what happened 35 minutes after that:

[21:00]  PkmnQ: observation: there is no grass under the chain chomps
[21:00]  PkmnQ: (at least in super mario odyssey)
[21:01]  PkmnQ: let's see other instances of chain chomp
[21:01]  ultsppi: https://www.mariowiki.com/Incoming_Chomp
[21:01]  PkmnQ: to see if it fits the grass theory
[21:01]  ultsppi: these once can jump
[21:02]  ultsppi: these can fit the grass theory if they travel to grass
[21:02]  PkmnQ: but there is the chain chomp in bob-omb battlefield
[21:04]  PkmnQ: i doubt that they would unchain chain chomp often to let the grass regrow, considering how chain chomp jumps over the hill
[21:04]  PkmnQ: this would be massively inconvenient and i think they would rather let it absorb the grass
[21:04]  ultsppi: do we have a conclusion
[21:05]  PkmnQ: the only theory left is the eye theory from earlier, that they do still photosynthesize but through the eyes
[21:06]  ultsppi: or the mouth
[21:06]  PkmnQ: https://mario.wiki.gallery/images/b/b7/Mk64rainbowroad4.png i don't know the rainbow road lore
[21:06]  PkmnQ: does this rulle out photosynthesis entirely
[21:06]  PkmnQ: i don't knkow where rainbow road is located
[21:06]  PkmnQ: if it is sometimes daytime then it is still believable, however if it's all the way in space then we have a problem
[21:07]  PkmnQ: i think this rules it out entirely https://mario.wiki.gallery/images/0/0b/Chompy.png