I have the 020 and a version of the 022 and I also struggled to understand why the 020 did not accept regular 0.5mm lead even though 0.020” = 0.5mm, mathematically.
The 020 Leroy lead is almost exactly 0.5mm diameter and may predate thin lead ISO standardization. Per ISO standard, 0.5mm lead is actually 0.565mm +/-.
Noting that 0.565mm = 0.022”, it appears that there was an adjustment by Leroy from the early 020 model to the later 022 model to conform to the ISO standardized lead size coming into production from the likes of Pentel and others.