Not how I was taught it, although the argument about thin aerofoils is true - it doesn't explain why high speed aircraft don't simply have deep chord, thin stubby wings (apart from the horrendous tip losses).
The critical Mach number is not the absolute Mach number, it is the local Mach No, perpendicular to the leading edge of the wing. So, if you resolve the triangle of velocities, for the "ambient" Mach number to be that parallel with the wing centreline, then we get a lower apparent Mach number perpendicular to the wing leading edge. (It's a bit like calculating crosswind component). So, the greater the sweep, the lower the perpendicular Mach number for the same flightpath Mach number.
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