Opportunists are the ones who will sell a 0day to bad guys. Or who will drop a 0day publicly to promote their services. And they’ll fight tooth and nail against any actual legal obligation to engage in responsible and coordinated disclosure, because they make more money without that.
As soon as a patch is committed, the clock starts ticking, the exploit will be discovered by reverse engineering recent commits. The commit was made on April 1st, Xint disclosed it on the 29th. If the Kernel Security team had wanted to, they had 28 days to backport patches in the LTS branches...
So, I wouldn't put any blame on Xint there.