@panneqaha
It does take some research, given not being familiar with 「異世界の主役は我々だ!」, but given that isekai is a part of their name... (We are the main characters of another world!)
That said, without having determined that by following the link to their youtube channel and copy/pasting from their "about" tab into a translation utility...
The story so far gives no hint that either of the two MCs are not from the Demon World.
The other argument would be that since Mairimashita! Iruma-kun is an isekai, any parody collaboration would also be an isekai.
That's also kinda shaky.
If the MCs are from that world, it being a spin-off from an Isekai should be irrelevant.
This is only the first installment.
Their may be things coming up later of which the folks deciding how to tag this are aware of which would justify the Isekai tag.
Then again, maybe not.