Is it not blindingly obvious that the leather is coated with acrylic paint? Full grain leather is only ever dyed. Stained, if you will. Never painted over with an opaque acrylic paint.
I hate to use Louis Vuitton as an example since I am revolted by that brand, but they use full grain leather and they are easy to google for. Notice the waterspots. Notice the transparency of the leather. You're looking at mildly dyed leather, not a thick coat of acrylic paint on leather.
When oiled, it takes on a deeper hue but the transparency remains:
Also, the flawlessness of the surface of those Filson leather should tell any idiot that the leather was sanded. My strap is about 1m (I did not measure, just estimated) and the leather has none of the natural creasing common to cowhide. It is actually more flawless than calfskin. Normal aniline dyed cowhide would show a hell lot more "surface interest".
So Filson uses sanded, acrylic-coated leather.