lesschumras, just a limud zichus – it’s very possible that these people do not realize there is someone davening SE behind them. It’s happened to me more than once.
notpashut, there is one other time that the phrase “one’s sin is “too great to bear”” is mentioned: when Cayen killed Havel, he asked Hashem “is my sin too great to bear?” And, as you said, the SHULCHAN ARUCH says about one who talks during chazaras hashatz his sin is too great to bear. (not to say that at other points in davening it’s OK)