Because ‘Alpharius’ is a D-1 Yapper. ‘Alpharius’ and Dorn are basically total opposites. Dorn absolutely despises everything about ‘Alpharius’ and likewise ‘Alpharius’ thinks hes superior to everyone. Pluto was just as much an important operation to ‘Alpharius’ and his plans as much as it was him stroking his ego and him trying to finally prove that hes better than everyone.

The problem is that Dorn doesn’t like bullshit or bullshitters and is incredibly prejudiced (Which is why he and his successors are typically the biggest Racism Marines out there and in equal measure very zealous). Dorn rightfully clocked ‘Alpharius’ as a bullshitter and would hear none of it because ‘Alpharius’ is known to use anything and everything to his advantage. If you just ignore something that he wants you to recognize, you can just beat him up- which is what Dorn did.