It was through the use of Equivalent Exchange that he earned Wizard, and Alchemist and Sex Maniac, both of which provide corrections to intelligence and MP, were not unlocked until afterwards, not to mention extra class slots, on top of significantly higher levels. His MP and INT correction is far higher now than it was then, and it is a skill that first uses all MP up before converting HP left over. The Thief he used it on initially was level 3, and the Explorer he used it on here was level 25. Thief gets only a slight correction to Agility, while Explorer only gets a small correction to Endurance; the Explorer has more HP, but compared to the MP gap he's jumped up, it's nothing like when he first used it. Levels do not matter if they are not in classes that provide corrections for the stats involved. That may be what you don't understand- that, and that class corrections only apply if equipped; most people only have the one so can't do something as powerful as he did there, unless they level up as a magic class to begin with.
Durandal is max point sword, yes, but no, those were not added in this scene. Durandal has, since its inception, even in the web novel version, had it.
Specifically, the list of effects it has are:
9x Attack Power, HP Absorption, MP Absorption, Incantation Interruption, Ignore Level Gap Modifiers, Ignore Defense
Fragarach, the tier below it, has:
5x Attack Power, HP Absorption, Incantation Interruption, Ignore Level Gap Modifiers, Ignore Defense
As stated, more points gets him more effects in the summoned sword. For Ignore Defense though, at least, there isn't much difference between the two topmost Weapons, though the lack of MP Absorption is why Fragarach is only really seen the one time, given that is his primary use for it.