My understanding of Type Ia supernovae is that they are expected, in most cases, to destroy the white dwarf(s) that went in to them, leaving behind no high density remnants (i.e. no white dwarf, neutron star, or black hole). Black hole/black hole collisions are expected to leave behind a black hole, of course, with less mass than the sum of the masses of the black holes that went in to the collision. Do we expect a kilonova to leave behind nothing but gas and radiation or some kind of stellar remnant? If it leaves behind a remnant, what class to we expect it to be (white dwarf, neutron star, or black hole) and what mass? The addition of mass seems redundant, but the dividing lines between the masses of these objects are based on upper limits on the mass of the less dense class (Chandrasekhar limit for white dwarfs, Tolman–Oppenheimer–Volkoff limit for neutron stars), and don't actually apply as lower limits for the mass of the high density class. For example, neutron stars are sometimes referred to as giant nuclei, which would put the lower limit of their mass at $1$ or $2$ atomic mass units, depending on whether the presence of a neutron and stability to radioactive decay are requirements. Yes, those times people are being poetic due to the difference in stabilization mechanisms (gravity vs nuclear forces), but the point remains that it may be possible for a neutron star to be theoretically stable at less than $1.4M_\odot$. The only lower limit to black hole mass I know of would be the lifetime limit from black hole evaporation.
Put another way, do we think the density at any point in either of the inspiraling neutron stars gets high enough to form an event horizon? If that happens, this seems like a plausible way for producing black holes that are smaller than those produce by core collapse supernovae, kind of like how critical mass for atom bombs can be achieved by either bringing together enough fissile material or compressing the available material enough (e.g. the fat man detonation mechanism vs the little boy one).