Kagi is also building their own index at the background, and mixes these indexes as you search.
When I search Kagi for "Hacker News", results start with this fine text:
65 relevant results in 1.09s. 47% unique Kagi results.
So, other indexes are fillers for Kagi's own index. They can't target their bots to places, because they don't have the users' search history. They can only organically grow and process what they indexed.