What people are pointing out is that you're not arguing for having more information. The "severity" is a marginal zero information on top of the priority. There is no context where you would want to use "severity" instead of priority.
So...
GIVEN that priority trumps "severity" 100% of the time,
AND GIVEN that priority is already being recorded,
THEREFORE "severity" has no use, and shouldn't be recorded.
What's happening in the post is that someone wondered why there were two fields, and made up a justification for the second field instead of realizing it was a mistake to have two.
You can see the mistake happening whenever people choose a word that ordinarily refers to the concept of importance as their label for "severity", while simultaneously acknowledging that "severity" doesn't affect the importance of the issue.