I have come to the opposite conclusion - YouTube does not reliably include spoken word/transcript matches in search results.
As I wrote in another comment, I've run tests where I open up the transcript of a video using these steps (https://kb.swtc.edu/page.php?id=90230), copy a few of the words, then perform a search using those words across that channel, and the video I copied the words from doesn't appear on the list of results.
If Google decides that it's going to revamp YouTube search to include spoken word/transcription matches, that would make VideoMentions Search irrelevant– and I'd be okay with that!
In the meantime, I think this is a useful free tool for quickly finding spoken word matches within specific channels.
Hey @tomatowurst! You can search within the spoken words/transcript of a single YouTube video by following these steps: https://kb.swtc.edu/page.php?id=90230
Also you can search individual videos by showing transcript and pressing ctrl-f