I have never seen a matrix result show up in google though.
the ones on my homeserver aren't, and searching for the domain of the homeserver shows that users are in rooms on other homeservers/networks.
Under "Room Settings -> Security & Privacy -> Who can ready history?" do you have "Anyone" set? I believe this controls whether it can be indexed or not, as if you have it on any of the "Members only" settings, their bot needs to be in the room for it to see history.
Disclaimer: I'm fairly new to administrating a Matrix server so I could be wrong, but the logic seems to make sense with my suggestion.
Ah, that setting was not ticked. Interesting. I might try this on a different room.