Any of the HDPE products lack the stability to prevent flex and properly "back", unless VERY thick. The ability for anything, even 5200, to stick to them for long is what really makes them a poor choice though..
I had a customer use 3/4" HDPE for backing plates, Sea Board (same as Starboard) and in one season 3 out of 7 were known to be leaking. They provided no support and the flexing when turning the handles cause the seal to leak.
A small batch of boat builders tried this over the years, mostly small power boat builders, and I suspect also ran into problems. At the last boat show I did not see any of them using it for backing blocks any more, even ones I'd recalled seeing in the past... HDPE is very flexible and you might as well be trying to bed a Teflon frying pan, sealant wise... Nothing really stocks to it...
Leave the cutting boards in the galley and use known time tested and proven methods below water to keep your boat from sinking or leaking...
Of course you don't need to take my advice here's what the manufacturer says:
, King StarBoard® ST, King ColorCore®, King ColorBoard® and King CuttingBoard® can not be glued using standard adhesives
. Products like 3M’s 5200
work well as a water sealing caulk but will not adhere King StarBoard® to itself or other materials in a permanent bond.