Surely they just want to avoid straining their database so they put some "performance hacks" into their database instructions that they Ab/B-tested to "work" for 90 percent of people or something.

Meanwhile they could have just returned the titles of all your videos you have ever watched as a list and let your computer do the heavey lifting by searching through that text on the frontend only to fetch thumbnails and such for the final matches. I have a webservice with a table of 4000 lines or more and I can search it quasi instantly on my smartphone with a simple Javascript script hooked up to an input field.