Is this another lesson in why you should not invent your own crypto system?
They didn't actually invent their own crypto system. They used the scheme devised by Swedish cryptographer Rolf Blom, know as Blom's Scheme. Which is a form of "threshold secret sharing". It has been known for quite some time that the system falls apart once a particular number of keys are known.
I assume that each device should have it's own keypair, but it is only my assumption. I recall that HDCP somehow "does not work as intended" (whatever that means) when both devices have same key. And as for the ~50 number, that seems likely, I vaguely recollect that the required number was 40 (probably for 50% chance of success), but I don't remember the exact details and assumptions for this attack, so I take "at most thousands" as safe overestimation. Also I expect that time complexity of this attack (which is probably not exactly fast, as it entails solving pretty large system of equations) decreases with more known keys.