No collision of 2 surfboards is going to loft debris high enough to be in a stable orbit. If the common point of orbit is 500km, all of that debris will de-orbit. The only propellent on those things is krypton, so if they collide it is only going to be mechanical effects on the debris. In my search I have not found a single reputable source claiming starlink could contribute to kessler syndrome, only clickbait articles.