Yes, it's critical, but is the criticism unwarranted? He is questioning why one of the most senior maintainers is directly contradicting one of the most central "edicts" from Linus on the kernel development: Don't break user-land. In the message, Linus directly quotes Alan as arguing that breaking userland is ok.
Of course Alan was/is free to disagree, but he should have known very well that Linus would never let that fly. Not least because Linus had told him it wouldn't, and he kept pressing for it.
What was the alternative? From the outside, it looks like Alan repeatedly avoided doing what Linus told him needed doing. Linus could not have backed off without sacrificing the guarantee of not breaking userland.
