Yes but the point is that it was thought this isn't something that was to be required - and had the system been correctly implemented (and handled by the crews)

Yes but the point is that it was thought this isn't something that was to be required - and had the system been correctly implemented (and handled by the crews), there wouldn't have been a problem. I don't think Boeing was intentionally "hiding" the existence of such a system, they just didn't consider it critical enough to bother the crews with it because it was supposed to come into play only in a very unusual flight regime - manual flight, flaps up and high angle of attack. There was no special "MCAS training" needed because any failures fall into the "runaway stabilizer" emergency procedure. It just didn't work out exactly that way, unfortunately. If I recall correctly, the system has been mentioned in the documentation - not prominently and not something the crews would have been forcibly made aware of, but given that the plane certification depended on it and it had to be certified, they couldn't exactly "hide" it from the regulators. Keep in mind that the procedure for the stabilizer problem is the same since 737 has been first introduced in the late 60s. Had it been correctly followed, likely neither of the crashes would have happened, MCAS or no MCAS. The Lion Air crew didn't identify the problem as a possible runaway in the first place for whatever reason and the Ethiopian crew made several (understandable, but still) mistakes, such as cutting the trim system out while heavily out of trim, not reducing the excessive speed and then finally re-activating the electric trim (which is against the procedure) when they couldn't re-trim the plane manually due to the resulting excessive forces. Which has directly caused the crash once the MCAS has re-activated.