I imagine they just thought "we wanted two guys carrying him up" and they already had graphics for Mario, and/or thought it wouldn't look as good if there was a different character used just for the intro.

@DragonLordRyuoh I would take it as a pitch of salt. Gameshows even to this day get the questions from writers and probably a theory from said writer. There wasn't any official references from Nintendo (who only used Mario in marketing and made numerous references that Luigi debuted in Mario Bros.), Coleco or Atari (who did the ports). Maybe they got the idea of two Mario brothers later down the line by this, who knows. After all Mario himself was called Jumpman until the US release of Donkey Kong (some arcade material still had the Jumpman name so it was a fairly late change) and by then became his in game name for DK Jr.

Besides Nintendo wouldn't talk much about this game due to all the legal drama that it caused (Cliff notes: Nintendo outsourced Donkey Kong to a TV camera company think it was Ikegami who they had used in the past for programming, Nintendo wanted to make this but couldn't get the company so they hired some engineers to reverse-engineer DK for this game, said engineers eventually lead to Intelligent Systems one of the earliest 1st party developers, TV camera company figured out and took Nintendo to court, a twist said TV camera company went to Sega to make Congo Bongo and Zaxxon then bowed out of gaming development). Like DK, this is why until recently Nintendo only re-released the NES versions.

Seeing the arcade cinematics, we see what looks like TWO Marios carrying Donkey Kong and wondered if there was a glitch.

According to a trivia question on the 80s game show "Starcade", this is where Luigi actually got his shot before joining Mario in "Mario Bros."