'I can't win… even with my Progenitor Authority.'
Arthur didn't delude himself. He acknowledged it right away.
Kalvaro was a Mid-Tier Deity. Inferior Stage, yes, but a Mid-Tier Deity all the same.
Besides the vast gap in Existence Realm alone, Kalvaro also possessed a Unique Authority whose powers Arthur was unaware of.
'I can nullify it with my Aspect Manifestation, sure, but there's no way a Mid-Tier Deity only has one Authority.'
There were likely weapons and concealed artifacts Arthur didn't know about.
Most importantly, his most effective card against gods couldn't lower a Mid-Tier Deity's strength to a level he could properly fight.
At best, the current Inferior Stage Low-Tier Deity Arthur could drag Kalvaro down into that limbo between realms. But even then, he wouldn't win.
Not a draw, but an inevitable loss. If it really came down to a fight, Arthur would go all out right from the start, but only to escape. Escape after inflicting significant damage.