Keyword density is definitely important, but I would like your opinions on a video like this:

https://www.youtube.com/watch?v=Jl-QlXGTPlA&ab_channel=MINUTTIA

I agree with the SEO vendor.

If the podcast is on Google podcasts too, Google's algorithm transcripts speech into text in order to spot keywords and rank it on the serps.

Just like it recognizes keywords in blogposts, or innapropriate language in youtube videos.