To be fair, once Xint gave the heads up and the kernel team committed a patch, what was Xint supposed to do? Keep asking the kernel security team to backport patches for the LTS kernels?

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.