Neither the headline nor the writeup itself explains it. The fact that there are 500+ comments on this post and not a single one has explained why exactly this CVE needs to be fixed with urgency tells me that Ubuntu's decision is likely the correct one.

The explanation is everywhere: it can lead to RCE , researchers achieved it with mptcp but nothing prevents attackers finding another mechanism. Yes, it is hard to exploit it outside mptcp but nobody has demonstrated it is impossible, so we should be cautious and tag it as urgent