This paragraph was in the article you attached :
"Dr. Ellebedy’s team obtained bone marrow samples from 19 people roughly seven months after they had been infected. Fifteen had detectable memory B cells, but four did not, suggesting that some people might carry very few of the cells or none at all.
“It tells me that even if you got infected, it doesn’t mean that you have a super immune response,” Dr. Ellebedy said. The findings reinforce the idea that people who have recovered from Covid-19 should be vaccinated, he said."