I don't own a Smart TV, but without this step I'd be very suspicious that the TV would silently try to connect to open wifi networks, or perhaps some kind of hotspot like xfinity or AT&T that the manufacturer made an agreement with.

Won't they just use Ethernet over HDMI?

Maybe, but that would only work if there is Internet at the other end of the HDMI.

Oh FFS I just learned this was a thing, I can't even trust my cables anymore.

Be aware that your ethernet cables could also be powering stuff, so that's another cable to watch out for ;)

Watch out for the reverse too, ethernet over powerline is a thing!

I would skip the VLAN step and just block communication from the TV to the internet. That way you could still access media on other devices on your lan via DLNA or something

What happens when you want to use Netflix, Amazon prime, or an external Plex server with the smart TV? You NEED internet access for those

Separate HTPC box that you trust?

My ideal TV has power and video in and nothing else. Basically a monitor. I have a separate box for content and another for input switching.

FYI you can't use an internal Plex server either. The Plex client is unable to operate without an Internet connection.

Chromecast or Roku