Oh in just a couple of blocks, it'll give you a somewhat reasonable estimate for asking about every confirmation count other than 1, but it could take several hours for it to have enough data points to give you a good estimate for getting confirmed in one block (because the prevalent feerate is not always confirmed in 1 block >80% of the time) Essentially what it does is just combine buckets until it has enough data points, so after the first block it might be treating all of the txs as belonging to the same feerate bucket, but since the answer it returns is the "median"* fee rate for that bucket, its a reasonable answer right off the get go.
Do you think it would make sense to make that 90% number an argument to rpc call? For instance there could be a default (I would use 80%) but then you could specify if you required a different certainty. It wouldn't require any code changes and might make it easier for people to build more complicated logic on top of it.
*It can't actually track the median, but it identifies which of the smaller actual buckets the median would have fallen into and returns the average feerate for that median bucket.