No.
It was true in 1960. Back then, good wide-angle lenses required mirror-lockup, eliminating the utility of an SLR. Since that time, SLR manufacturers (including Leitz) have poured a great deal of science and effort into making high-quality wide-angle lenses. Probably 99 percent of the imagery you see today comes from SLRs, many of them using very wide lenses.
It is simpler to design a wide-angle lens for a rangefinder. That doesn't mean the more complex SLR lens -- using modern coatings and modern glass pouring techniques -- will be inferior. Any good really wide angle lens ... like a 21mm or wider ... is already very complex anyway, using 8 or more elements.
Probably the best wide-angle lens I've ever used is the 24mm f/2.8 Nikkor, which cleverly used a floating element to overcome some of the problems associated with SLR lenses.
Lots of people strongly believe there's a rangefinder advantage to wides. It's possible they are correct, but I've never seen it in practical use, and I've never seen a side-by-side comparison.