Milius was retconning with that explanation, plain and simple.
You're able to read his mind?
Writers and directors do it all the time. Whatever he might have "really" meant, all we, the viewers, have ever had to go by is what was said in the released version of the movie. If he meant something else, then he should have put that in the movie.
You're saying this about a movie that also features a silenced .357 magnum revolver, for pete's sake. Do you really imagine for an instant strict technical accuracy was a priority?
Harry Callahan using light loads in a heavy pistol for better control makes more sense than a lot of the other gun nonsense that made it into those movies. Using a .458 magnum bolt action rifle as a counter-sniper weapon comes to mind...or would you prefer we discuss the harpoon gun?
Yes, and given those over-the-top powerful weapons, doesn't it seem a bit incongruous that Harry would then reverse this trend toward maximum power in his primary weapon? A light .44 special -- meaning
literally the .44 special cartridge is
eminently controllable in the .44 special N frame revolvers S&W had been making since the early 1900s. It would make no sense to go to a 6.5 barreled Model 29, and give up a huge degree of concealability and speed on the draw, to say nothing of how much more the Model 29 would have cost a poorly paid cop, in search of a truly negligible amount of extra controlability -- all to end up with a less powerful handgun than numerous other SFPD cops carrying .357s. It makes far more sense to suppose that Harry really was carrying magnum loads, given the umpteen billion other times he refers to his handgun as a .44
magnum (and touts it as "the most powerful handgun in the world" -- a strange thing to say about a .44 special), vs. this one
single mention of a "light special." Given that, and given the writer's own statements on the matter, I don't see why there's even a debate.
But as for this trend I see on this board of postulating that Harry, today, would have a .454 Casull, .475 Linebaugh, .500 S&W magnum, etc... I think that's taking things way too far. The .44 magnum was picked because it was the most powerful handgun generally available back in 1971 (and it wasn't even very available back then -- they weren't selling well and S&W had, IIRC, stopped making them; in fact they originally meant to give Harry a 4" bbl one, but they simply couldn't find one, so he got the 6.5" bbl), but except for being beefed up to mitigate the felt recoil of the .44 magnum, it really wasn't any different than other S&W large frame revolvers lawmen had been carrying for decades, and so, not that much of a stretch to imagine a cop carrying.
A slow-to-reload, 5-shot, single action hunting revolver, on the other hand... That's straining suspension of disbelief just a wee bit too far.