This does sound odd.
One of the most obvious things that City players do is receive the ball across their body, even if it's the wrong foot. I'd have thought Laporte, when a ball comes from the left, would receive with his right, and then play with the inside of his left boot, so it's then easier to play it to the right. (Short passes he should be able to play with either boot, but we don't expect to see him playing the difficult long passes out to the left wing, which is why he played in the FA semi - he was there to play the balls behind to Torres on the right).
If he's receiving with his left all the time, you're correct, that is going to make life difficult for him.
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Just had a watch of the first 30 mins of the match. At the very start he does receive the ball on the outside of his left foot, control it that way and hit it back with the outside, but I didn't see him do it again. It was in the first five minutes so not surprised Izzy Christiansen noticed it - if you're starting the match wondering how he'll play on the "wrong" side of defence, and that's the first thing he does then it does make you worry.
However, after that, most of his passes to the left are with his right foot, as you'd expect. His chalkboard on whoscored doesn't tell you which foot he's playing with, but it shows that his passes are split almost exactly 50% left, 50% right. As a comparison Stones made something like twice as many passes to the left as he did to the right against Croatia.
I'd have thought most of Laporte's passes would be left to his keeper, Pau Torres or Rodri, the fact that they only make up half of his passes, suggests he actually has a bias to playing to the right, which is what you'd expect with a left footer.