May 31, 2021 is (just) outside the 2-year period ending on May 31, 2023, so claiming the exclusion on the May 31, 2023 sale is not barred by the once-every-two-years limitation. Had the sale taken place on May 30, 2023, claiming the exclusion would not be permitted.
Section 121(b)(3) Application to only 1 sale or exchange every 2 years
Subsection (a) shall not apply to any sale or exchange by the taxpayer if, during the 2-year period ending on the date of such sale or exchange, there was any other sale or exchange by the taxpayer to which subsection (a) applied.
(Subsection (a) is the exclusion.)
