I'm having a hard time understanding why (near the start of the video) someone is shown stepping among (& nearly ON) some of the snakes...

That must be amazing to see in person, so many at once. I'm happy when I just find ONE in my yard, as I did yesterday while trimming shrubs by hand. I suddenly realized I was being watched intently by a 2' ribbon snake (similar to a garter snake). I didn't try to touch her & she was giving me the "stink-eye"- like "I'm WATCHING you, don't try anything funny!" I say "she" because the body was rather hefty & looked like she might be gravid. I hope so. At any rate she looked healthy- it was delightful to see her enjoying my yard.