"Originality" is a hard thing to pinpoint when it comes to games. New mechanics are exceedingly rare, and usually involve some mix of what came before.
I can see the similarities here, but I'd argue Driller presents a new experience. I didn't know about this C64 game while judging, but the damage mechanics are different, the art is entirely new, and the levels are arranged differently (at least from a cursory glance). That's enough for me to call it original.
I wouldn't call it "plagiarism" unless it's pulling assets from other games or making an exact clone.