That's why I built VideoMentions Search. It pulls in the transcript for every video within the channel and date range you selected and shows you any and all matches.
Also - even when a Google search does work to find a video based on spoken words, all you're given is a link to that video's page. You can't see all the times that keyword was mentioned within the video and click on one of those mentions to jump to that exact moment in the video, like you can with VideoMentions Search.
If Google/YouTube added those capabilities, then VideoMentions Search would be irrelevant. In the meantime though, I think it's a useful free tool for quickly finding spoken word matches within a specific channel.
Thanks for checking it out!