Lyot may be right as far as a handful of Mars craters go.
But.....there's that word [but]...but
you don't need a mainframe computer
simuilation to know that a Kuiper Belt
object, redirected to hit Mars, with
1000's of trillion of metric tons worth
of frozen gases [N2, CO2, CH4, NH3, CN,]
will flash into superheated vapor upon
hitting the Mars surface.
Most of that gas and vapor will escape Mars gravity very quickly, but with proper calculations the Mars atmosphere could be enriched by between
100-1000 millibars worth of atmospheric
[greenhouse] gases.
Mars will still be very cold [you'll need a Parka: Think Alaska!], Mars
atmosphere will still be poisonous [Oxygen masks required outside of dwellings], but you won't need a
pressure suit.

Jacob