There aren't a lot of YouTube videos with Swahili subtitles. Some videos have hardcoded subtitles, which your search obviously won't find.
Would you consider maintaining a manual list of hardcoded videos if someone was willing to contribute/maintain?
e.g. here are some Swahili videos with hardcoded subtitles:
https://www.youtube.com/playlist?list=PLA5UIoabheFMUEsFNXr4ZGNam4-gEEZ1Q