I would not interpret your case as severe according to this grade. The specification “some assistance usually required” seems like they mean your reaction is so bad you need help eating/washing/using the toilet, which I assume was not the case for you. While—especially for a young healthy person—staying in bed all day is a “marked reduction” of activity, there’s still quite some room for further worsening before you’re at a point where it’s life threatening.
While the wording could be more clear, if my interpretation is correct I would agree this is an OK grading to use.
The preprint for the article on cognitive decline due to long COVID was shared in the LessWrong Telegram group last October. I looked over it at the time and wrote down some notes, which I will reproduce here. Note I haven’t looked through the final version of the article to check if things still match up.