It was a bug in that version of the distro IIRC, trying to install Steam would instead try to install the SteamOS desktop environment (or something along those lines). It has since been fixed to actually install the Steam client.
Obviously it was a bit silly he typed “Yes, do as I say” after seeing the message, but he was also literally following exactly what all the online guides said to do (other than the “Yes do as I say” part). Luckily it’s fixed now but I do think it was a really good demonstration of what the video wanted to see: “What might the average non-techie gamer face using Linux?”