I think it's very simple. I don't see how anyone who accepts judo as a martial art can deny that sumo is one. Sumo is essentially judo with a slightly emphasis (from judogi to mawashi), no mats, and full strength, non-gloved atemi.
To my knowledge, there has never been a death on the dohyo in the 100+ years of modern sumo. Nonetheless, for his entire active career, Yokozuna Wakanohana III kept a secret will, in the event that he was killed in the ring. Kitanozakura, a currently active rikishi, likewise apologizes to his family before every tournament, as he goes up on the dohyo prepared for the possibility that he could die. That shows how seriously the wrestlers take what they are doing, and also provides a clue to how dangerous it feels to them.
Kimura Shonosuke and Shikimori Iinosuke, the two top gyoji (referees), wear a dagger in their obi. The dagger symbolizes their willingness to commit seppuku if they make a wrong call. In actuality, of course, they are not expected to commit seppuku, but whenever one of the two top gyoji have one of their decisions reversed, they submit a retirement application. (This is almost always rejected by the Sumo Association, unless the gyoji has truly lost his ability to get the call right 99% of the time.)
These are things that are not a part of the pomp and circumstance of sumo -- they happen away from the public eye. But they represent the dedication and commitment of those involved in sumo.
I practice a classical Japanese sword art, which is of no practical use outside the dojo. And yet I would say (and I think most would agree) that I'm practicing a martial art. The rikishi of professional sumo (and their oyakata, who continue to train) practice their art with no less dedication and seriousness. It is combat effective, and they put themselves to the test for 15 days six times a year showing that. Frankly, I'm surprised the question has even been raised. Particularly by you, Mr. Palombi, as I know you enjoy watching sumo every basho.
Josh Reyer
Swa sceal man don, žonne he ęt guše gengan ženceš longsumne lof, na ymb his lif cearaš. - The Beowulf Poet