It also says that oxygen toxicity is a possibility only above 21% oxygen. In fact, with regular air there are way too many things that have already gone wrong if you are at 65 m depth (or even 50 m).

It's clear from reading the document that convulsions at "shallow depths" refers to the case of breathing 100% oxygen, where 1.5 m difference is the difference between <1.6 bar (safe) and >1.7 bar (absolutely not safe).