Seems simple to me. You are supposed to use Siri search in YouTube. The thing that got delayed was the Siri API, not the bluetooth keyboard support. YouTube will be expected to incorporate Siri API. Alternatively another App Developer will create a front end app that uses Siri API to search and return YouTube content.
This should all be sorted in a month or two.
Will Google actually allow YouTube to incorporate Siri?
That seems like a long shot to me.

Isn't that like
asking Apple to allow Google access to the ATV4
remote's microphone in order to use Google Now
voice search function?
Besides..... Siri on iPhone could never understand it
when I searched for "Karadaglić guitar" or some
uncommon names. It only gets it right if I pronounce
it in a weird English way, but not if pronounce it
correctly in the foreign language. Strangely,
Google Now voice search has not problem finding
him when I say "Ok Google, who is Karadaglic"

It even starts saying a short biography of the guy.
(this is on my Galaxy Note 5)