While that's true, the construction materials and methods used with cameras up to- and including the M4 used brass rather than steel or zinc found in the film models thereafter whether made in Canada or Wetzlar. Even the M5 has a pretty hefty "metal alloy" that is subjectively heftier than the M4-2 and following models.
Up through the M5, these cameras were all handmade in Wezlar. After that, Leica introduced steel gears and other more "mass produced" (relatively) parts to keep costs down, though the later cameras were still hand assembled and calibrated. I have read that even the Canadian built cameras went to Wezlar for final assmbly and calibration, though I have been unable to confirm this.
Does it make a difference? It depends whom you believe. The legacy film cameras have a reputation of being "smoother" to operate than the later ones though this is very much subjective. It's hard to know whether the older stuff is smoother only because it's older and that parts have worn in more completely. In my direct observation, my own 1961 M2 is smoother to advance than a just-serviced M4-P I played with in a store. Then again, it feels smoother (to me) than my late 1960s M4.
In no case are any of these cameras poorly built or badly engineered. With the exception of the early M6 meter debacle and some of the electronics issues of the M7s, Leica M bodies have always been top tier, regardless of model or construction methods.