This part looks nothing like every other rendition of tenna, even in the FMV intro itself
.

But toby used it to showcase the chapter 3 boss teaser. Why would he do that? Why eould he even include it at all?

Because it's the closest he could get to showing the real chapter 3 boss, the knight, without outright spoiling it.

People will say "why the fuck would the knight be on the TV" and the answer is simple.
Tenna himself says "and then the knight appeared", when talking about how the knight. But as we know the knight didn't even physically show up until the very end of the chapter, tenna even complains about this fact; the literal theme and title of the entire thing is LATE KNIGHT. So how did the knight "appear" for tenna despite not actually being at the house?

It appeared on the TV.