And the list both you and my post quoted is not "supposed" source but it IS the source.

I think that your attitude (to have doubts instead of just believing everything presented in front of you) is very important to study, though.

But your idea is far-fetched, in this case. Besides, what you suggested has no consistency - if you say katakana came from Roman alphabets, why you need to mention Greek alphabets or Phoenician (which is the origin of Roman/Slavic alphabets) ones?