Yes, the web version is made with SDL (and OpenGL).
On Android and iOS I found the touch input latency of SDL to be too high, so I use the native APIs there.
I use the native OpenGL API on all platforms.
I did make my own renderer. A renderer for PewPew is not too hard: culling is simple, no lights or shadows. Hardest part was probably drawing 3D lines :-)
What is a little bit annoying is that any new effect, like Depth of Field or Lens Flare take several hours to implement, whereas in a premade engine my understanding is that it’s one click away and the quality can be better than what I achieve.
Making the renderer is an iterative process. I started with just drawing lines, then progressively added all the different effects (bloom, chromatic aberration, etc…). You can get a renderer in a week of full time work, but then there are endless improvements you can make. Even today the renderer of PewPew Live could be improved in a lot of ways.
It’s totally doable to do a 2D renderer with ray tracing lights. 2D simplifies many many things!