If the deceleration varied as much as the report said, then the passengers probably could tell. That was very different from a normal landing. If that wasn't enough, being towed to the terminal might have given them another clue.
> One extra thing going wrong here, in the wrong direction (anything that impaired braking or pilot reaction time) would likely have led to loss of life.
Well, it would have lead to the plane running off the end of the runway. What's past the end of that particular runway? A cliff? A river? Or just another 100 meters of grass leading up to a perimeter fence, then more flat ground for some distance beyond it? If it doesn't overrun the runway too far (say, 100 or 200 meters), no, that doesn't seem likely to lead to loss of life - not unless there's some specific hazard there.
Regardless, that's one to be considered another aggravating circumstance, not an additional failure. When deciding whether this was a close call, you can treat it as if every runway is wet and in a permanent tailwind and immediately followed by a wooden building full of schoolchildren.
And you don't put wooden buildings full of schoolchildren immediately past the end of runways.
Four hopefully independent things went wrong here: three computer failures plus less-than-documented braking performance. Additionally, there were at least two aggravating circumstances: wet conditions and a tailwind landing. Part of the investigation is to find out if there was a fourth or fifth system failure (poor pilot reaction time? Rubber level on the runway unacceptable? Either is plausible, but far from indicated by the report so far).
One extra thing going wrong here, in the wrong direction (anything that impaired braking or pilot reaction time) would likely have led to loss of life. Investigating near misses like this, and not only being exercised once five things go wrong and a hundred people die, is a sign of a healthy safety culture.