BTW, Ray Arvidson the team geo llaer at the start was a good surficial strat and seismic guy. He is one of our best bolide expert . He was the one who picked out the potentil drill sites based on what we know about the rock makeup. Muc of it is compact iliceous material with no shallow bolides. If there were any impacts with Fe/Ni or chondritic bolides, these arent anywhere nar the depth that the drilling is intended. The Si matrix stuff has a hardness notmuch higher than 6(They did a lot of early multi-spec analyses and energy dispersive X ray and MS ) So I think were pretty certain of the hardness and the bit resistance. Arvidson was a real stickler. ( i think hes the guy that invented the terms "Meteorites" and "meteorwrongs").
I think what you are responding to is the assumption that when I say that Mars is essentially a large asteroid and thus would have very solid iron below a shallow layer of rust-dust, that I mean that would only be the case if the bedrock was formed by meteor impacts.
While I suspect that meteor impacts have occurred, I am also considering the fact that soils on Earth sediment over time and are compressed and formed by various bio/geological processes, so we are used to assuming that you can drill down into the ground because our ground has been formed from organic processes that don't render the constituent metals and other elements completely solidified.
Asteroids, on the other hand, are devoid of organic processes and bathed in unfiltered sunlight so the photoelectric activity within them over time must cause their atoms to bond together in a very firmly interwoven lattice, hence the strength/hardness/density of asteroids/meteors.
I am speculating, admittedly, but I don't think it's a ridiculous hypothesis to consider that Mars' surface is a thin layer of rusty-dust, with a much harder iron-ore substrate that has been hardened considerably due to Mars' exposure to high frequency solar radiation and its lack of organic soil-building processes that would break up molecules into loser arrangements.
It sounds like you are more familiar with NASA info about Martian geology than I am, though, so I hope you will reply with some details that speak to my admittedly weakly-grounded hypothesis.