(Airplane founder here) Airplane isn't YC backed. Though interestingly my prior startup, Heap, went through YC and has tons of YC-backed competitors (Amplitude, Mixpanel, Posthog, etc).
Personally I like that YC remains agnostic to the ideas and is willing to back competitors because it ultimately means more great startups get funded. Later-stage investors care more about conflicts because being involved at the level of taking a board seat matters a lot more for conflicts.
At this point they've backed 1000s of companies; if they had to vet that entire list for conflicts to back their next batch it would be incredibly difficult. Also, given the stage they're investing at, tons of companies pivot and end up competing even if they didn't start out that way.
Not sure if this is what you're implying, but I think it's a mistake to think of YC as a monolithic organization that makes decisions by saying, "idea X is good, we should fund teams doing it."
More likely, each of the teams doing each of these startups interviewed with completely different partners who had no idea of the other startups even existing, and in that interview, they thought the founders seemed solid and had thought through their idea well, and chose to fund it. It's even possible some of the people doing these ideas came up with the idea after they got into YC (i.e. they pivoted) - some of the most successful YC startups were companies that pivoted mid-batch (e.g. Brex).
In general YC doesn't want multiple shots on goal in a specific market area. They want as many shots on goal as possible among great founders in general.
Personally I like that YC remains agnostic to the ideas and is willing to back competitors because it ultimately means more great startups get funded. Later-stage investors care more about conflicts because being involved at the level of taking a board seat matters a lot more for conflicts.
At this point they've backed 1000s of companies; if they had to vet that entire list for conflicts to back their next batch it would be incredibly difficult. Also, given the stage they're investing at, tons of companies pivot and end up competing even if they didn't start out that way.