I feel like this would have the opposite effect, and search engines will still see this as active user traffic and prioritize Reddit in search results, which will still drive more ad views.
This might be useful if Reddit was looking to sell, as bot traffic could potentially devalue the platform (like in the case of Twitter's buyout), but unless they're trying to change ownership, I don't think this will actually hurt them at all.
I'm not sure how federation does anything to prevent that from happening, though. They can still do that on your instance, from their instance.
At most, I suppose an instance could defederate from a troublesome instance that's doing this, but the more that happens, the more fragmented the Fediverse becomes, and it starts to defeat the purpose of federation in the first place.