From a quick look at it, it seems to be a "case 1" potential error, as the proof is done by hand. That is the warning is likely because it is a mathematical exercise first, and the code is for illustration and not for inclusion in production software.
"Beware of bugs in the above code; I have only proved it correct, not tried it." - Donald Knuth
Source: https://staff.fnwi.uva.nl/p.vanemdeboas/knuthnote.pdf
From a quick look at it, it seems to be a "case 1" potential error, as the proof is done by hand. That is the warning is likely because it is a mathematical exercise first, and the code is for illustration and not for inclusion in production software.