How might've "then" become "than"?

The OED says of "than" in comparative expressions:

Its employment as the connective after a comparative (= Latin quam, French que) is a pre-English development, existing already in West German.

How the conjunctive use arose out of the adverb of time is obscure. Some would explain it directly from the demonstrative sense ‘then’, taking ‘John is more skilful than his brother’ as = ‘John is more skilful; then (= after that) his brother’. Others derive it from the relative or conjunctive use of Old English þonne (then adv. 6), = ‘When, when as’, thus ‘When as (whereas) his brother is skilful, John is more (so)’. The analogy of Latin quam favours a relative sense.

Ayto's account would seem to match the first explanation, that it went from "John is better, then James" (if this structure seems problematic, it is similar to how we would say "the order is first John, then James") to "John is better than James".

However the OED says this is probably the wrong explanation and it more likely derives from "When/while/whereas James is good, John is better", with "then" substituted for "When/while/whereas" and the sentence order reversed to give "John is better than James (is good)", possibly based on people copying Latin grammar.

Reference

"than, conj.". OED Online. March 2021. Oxford University Press. https://www-oed-com.nls.idm.oclc.org/view/Entry/200120?rskey=MmukIg&result=2&isAdvanced=false (accessed March 18, 2021).