He (she?) wasn’t talking about English a-, but Italian. This site (in Italian) derives the a- in anormale from Latin ab-, and I would assume the a- in amorale in from Latin in-, etc.
Because this function of the letter “a” was used in Old Greek language,too(clearly Greek people didn’t use the letter “a”,but its counterpart “alpha”)…
On this general subject, the Lexicon Valley podcast may shed some light. They recently discussed the changes in the past progressive in English, and it turns out that the way to express ongoing action in the past has undergone several changes, with pedants always decrying a new Dark Ages when the new standard comes in. One of the phases it went through involved prefixing the verb with ‘a-’.
Just for info, I’m sending a cross-ref of this thread to the “Christ is Risen” thread.