You can argue that a partnership is better than another partnership, that's fair enough but to say of a one off game that we would not have got a clean sheet if another defender had played is laughable. You just don't know. Nobody knows.
It's utterly unknowable and hence an ineffably stupid thing to claim.
While I agree there's no way to know for certain whether we would have kept a clean sheet if Carra was involved, hopefully you agree that this absolutely does not mean that one is incapable of making an inference as to whether the observed result was better or worse than the expected result had Carra been involved and it's also not the case that the result provides no evidence supporting one or the other hypothesis.
For instance, take all the games played by Carra this year (weighted by the similarity to game in question) multiplied by the goals allowed in those games to get the expected number of goals had Carra played (or if one wanted to be more ambitious, one could use all the goal chances allowed multiplied by the average probability across a larger sample that a chance will result in a goal - since "goals" is likely to be a more stochastic statistic than "chances"). Since that expected value is really just the mean of a distribution of possible values, this gives us a probability that the actual result was better than the expected result. Obviously there's going to be some subjectivity in such a measure (i.e. - what games are more or less "close" to the current game) - so not everyone will come to the same conclusion, but that doesn't mean that attempting to come to a conclusion is a stupid endeavor.
For me, when I go through it, I see us conceding in that game more often if Carra was playing than not (he's been done more than once by reasonably mediocre players this year, so I don't think it's super unlikely that he would have been done at least once by Odemwingie or Tchoyi).
So - if your issue is with people using deterministic language when they should be using probabilistic language - then I agree - anyone who says that we
definitely wouldn't have kept a clean sheet if Carra played/the only reason we kept a clean sheet is because he didn't play is indeed a bit ridiculous. However, if your point is that we can't say at all whether the result was better than what we could have expected had Carra played, then I disagree.