Question: Is this a strawman

I would categorise this as a false comparison. The error concerns not the argument itself but what the argument is about. The Mexican border and the Israeli/Palestinian borders share virtually nothing on common other their actually being borders and, indeed, concerning any number of points, what is true of one is more than likely false regarding the other. Phrased differntly, this "argument" weighs AGAINST building the US/Mexican wall, precisely because the circumstances of the Israeli/Palestinian example are LACKING in the US case.