Who is Catholic?

[Philthy]

But according to this answer, the Church uses the common sense definition then and not the theological one.

catholic.com/quickquestions/is-a-catholic-in-the-state-of-mortal-sin-still-a-member-of-the-catholic-church-or-doe

I think you are mistaken - I see both being used.
Catholic Answers staff:

He remains a Catholic (ie Common understanding - the person remains Catholic by virtue of having been Baptized and Confirmed in the Catholic Church), but he’s cut off from communion with the life of the Church. Through repentance and the sacrament of reconciliation he can be restored to communion both with God(ie theological sense of Catholic Church - he is again in a state of grace) and his fellow Catholics (Common, active sense of being a Catholic (he can fully participate in Holy Communion).