A biopsy has shown that he is actually a cat" [...] "And how long will it be? And where is Midnight?"

"We could not find him [...]

I'm not a medical expert, but I think a biopsy involves cutting open a deceased body, so either they did not do a biopsy or they did find midnight. if they did find midnight and do a biopsy that means that he cannot be a projection, so that argument is superfluous.

9537227
"Biopsy - an examination of tissue removed from a living body to discover the presence, cause, or extent of a disease."
"a bone marrow biopsy"

I was also not sure, but the encyclopedia says that it is taken from the living.

9537288
fair enough, but then how do they think he is still a projection if they were able to remove some tissue?

9537292
As i already said on another website:

Projections is a very general concept. Some powers allow you to create corporeal projections that have not only density, but also other biological and physical parameters.

Endbringers created by Eidolon's power as an example.
And according to the author of the Worm, Danny could summon rats from nowhere.