I just now saw that you replied below. I can merge if you want.
I think you're right, I think the triangles denote a surface roughness spec.
If you think about what the flywheel surface does, it provides a surface for the clutch disc to mate. Accordingly, it needs to be very flat, not warped from heat, and free of surface imperfections.
Given both those thoughts I would consider looking for a new flywheel. I don't think cracks are just a surface phenomenon, like blue heat distortion or scratches would be-- they indicate that the metal has changed dimensions enough for the cracks to propogate.
Flywheel Specifications