One understanding or assumption I had, was that when Gleeson first said it, he was just exasperated. Only later did he realize that he meant it. I am basing this on ... nothing.
But also a second thought, based a little more on the story: His whole complaint is that life is brief and otherwise pointless unless you do something dramatic with it, and only then was there any sense of living. He set out to make art that would live through generations. But if he couldn't make art that was worthy of being handed down the generations, then he would at least live out a life of radical commitment to his ideals. Being so constitutionally opposed to being forgettable and pointless, he thought that chopping off his own fingers would itself be an act worth being talked about and remembered.
That's also consistent with the moment where he "likes Farrell again". Because for a moment, Farrell was exceptional and interesting, even if it was angry and destructive -- there was something worth remembering there.
Also, as a fellow thick-skulled man, a character that is bizarrely obstinate really makes me "feel seen".