M and L and C are abbreviations for "moveto" and "lineto" and "curveto" from Postscript (which SVG was based on). I thought pen up and pen down in logo were "PENUP" and "PENDOWN."

It looks completely ordinary to me--exactly the sort of thing I wrote in HP/GL2 stuff for plotters. Your only possible tool on a plotter is a pen, of course the language is based on pen movements.