Because it’s for a company that makes a high end blending appliance. The sponsor is what makes this odd. I don’t see Cuisinart or KitchenAid sponsoring much other open source code.
I had the assumption that efficient SVG rendering in Safari was critical and a UX bottleneck to work on from an Apple perspective, but maybe Apple hardware makes it moot.
Therefore for embedded devices using WebKit/Linux GTK/WPE ports we see a huge improvment with the layer based engine. Also macOS/iOS benefits though -- animations are even faster, and you don't easily run into problems, when animating parts of a complex document. It's shown in the video linked in the article at the end: > 20% frame rendering time improvement on an already quite efficient rendering pipeline (Safari / macOS using CoreGraphics on M1 machine).