IIRC it was an "optional safety enhancement" that airlines could buy and their logic for not including it by default was "well we display the outputs of the two sensors independently don't we? Why aren't your pilots paying attention and crosschecking the sensor readouts on our 21st century glass cockpit airplane like this was a B-52 with needle gauges then?"

What we do know is that they argued that the errant MCAS activation from a faulty sensor was "designed to" look like a stabilizer trim runaway (when the "rear wings" you see on the tail of the airplane start moving without pilot command) and therefore claimed that a "properly trained" pilot should have been able to deal with that since they're supposed to be trained for a trim runaway.

This is a garbage argument of course, because a trim runaway is in itself an emergency that threatens the safety of the aircraft, especially if it happens at low altitude, so why the hell should your supposed "safety" system be putting the pilots in that position to begin with? And if this wasn't a big deal, why go out of your way to hide the fact that a new system on the aircraft can effectively cause a trim runaway? Not to mention that Boeing is essentially victim blaming the pilots that died from their profit oriented decisions by insinuating that they were poorly trained in order to take the heat off their shoddy design, going as far as to say that the pilot training in those countries were not up to "American standards," basically "those shithole countries don't know how to fly our glorious American planes so it's their own fault!"

Finally, it needs to be mentioned that when Boeing had its own test pilots use a flight simulator to demonstrate what a "properly trained" pilot should be doing when MCAS misbehaves, the pilots used unconventional maneuvers that are not apart of the standard operating procedures of the 737 (i.e. not apart of pilot training). What's more, their own pilots lost more altitude in recovering from the failure than the pilots of the accident planes even had, so wouldn't that mean that by their own admission the accident planes were in an impossible situation, proper pilot training or not?

If they extended the logic they had about MCAS and the angle of attack sensors, the solution to all of aviation safety would be to tell the pilots to "just don't crash the plane." If Boeing had their way and applied the same reasoning to all aircraft systems, they would probably make TCAS, GPWS, RAAS, and the evacuation slides optional features too, since the need for all of those can also be negated by the pilot simply paying more attention to not crashing.

Plainly Difficult has an excellent video about the technical aspects of the 737 MCAS scandal and Boeing's botched response to it, if you're interested!