I've been studying it for over a year as the pandemic rolled out and the researchers observed re-infections that resulted in more severe disease and death. 90 days was the nominal figure. The study cited below includes laboratory quantitative analysis of neutralizing antibody titer to SARS-CoV-2. The half-life it cited at 73 days. That means half the antibody has decayed in a 73 day period. Re-infections were observed 90 days after "recovery", so susceptibility to infection is returning just a little beyond the 73 day half life.
Your link says absolutely nothing about mutations. Nor would the suggested phenomenon need a mutated virus for an additional infection, in the hypothetical scenario.
“The study cited below...”
And, you do not link to a study but a popular article meant for lay people.