From the YT description:

"*** This was done by a professional and only for demonstration purposes, to show how viscous the molten rock is ***

***Please consult with professional guides who are safety conscious if you are thinking about such a trip. Much of this new land is highly unstable and large chunks can fall into the ocean without warning, or you can be on top of an emptying lava tube and that can collapse. With the intense heat, sharp rocks and constantly changing lava flows, you can easily get trapped or hurt and thus should be careful and go with professional guides. ***"

So I wouldn't call this person a dumbass for doing it.

I would.

Seems like there are other things to poke lava with that don't contain a piece of your anatomy.

Dumbass and a coward, should have tapped it with their genitals and not a rubber encased foot.