The real deal is this:

Google's search scope is "the internet"

Youtube's search scope is "videos uploaded to it's site"

How you define what 'is' a search engine or what 'has' a search engine is moot if you talk about the 'scope' in which a search is performed and then it's 'method'.

If I created a search "engine" thats scope was the internet and it used the same 'method' as google then it would give me the same results as google. Same argument goes for Youtube.

Youtube just happens to be a special case in that it is a search on a scope of video files which are a subset of it's own website. IE it searches specific conent uploaded to it's site which allows it to get added meta information from an otherwise visual medium, IE how does Google know what's in a video.. Youtube know because it asks you to tag it.

Same with Slashdot's search, It is a search on it's own content, submissions, comments etc.

Just because Google can search "outside of it's internal scope" (however that may be defined) doesn't make it any different or doesn't make Youtube any less of a search engine.

The point thus stands that this is pretty damn spectacular that Youtube which is only searching a small subset of data is being used more than Yahoo which is used to access in theory the same or a similar scope to Google, ie the internet.

How accurate the method on counting how you rate Youtube higher than Yahoo is, is open for debate. However assuming they are using the same metric they always did, according to that metric- Yahoo was above Youtube, and now it isn't. I would therefore say this is significant even if it is (without knowing exact details on how the ranking works) potentially somewhat crude.