Now, the world of sumo is a bit of an oddity, so I would temper this - he was only a high school sumo competitor, from what I can gather he never went to a heya or enter the banzuke so he hasn't fought any tournaments.
In sumo, there are six pro divisions which are contested by athletes who are part of professional sumo stables called heya and your rank in it is determined by your win-loss record. Divisions 3-6 get flat salary allowances based on the level that they are at and usually live on site. Divisions 1-2 get prize money and bonus earnings for special prizes, titles, etc. and usually live away from the stable coming in for training. Often a division 3 rikishi will serve as an attendant to a higher ranking wrestler in the same stable to gain experience of life at the top level.
Because of the grind of lower level sumo, often wrestlers will choose to bypass the early part of sumo and look to compete in other non-pro tournaments, and competing and ranking well at these will entitle a wrestler to enter professional sumo at a higher rank, usually in division 3 (the highest allowed is Makushita-15, or the 15th rank in division 3, with two wrestlers holding each rank). These subdivide into three types: amateur sumo, corporate sumo (wrestlers often representing large businesses as well as things like the military, navy and air forces) and collegiate sumo (high school and university tournaments). You can usually tell these wrestlers because they appear in the top 3 divisions but do not have a topknot yet, because these amateur, corporate and collegiate competitions do not require the strict lifestyle rules applied to professional sumo and stable life - therefore often wrestlers who are unsure of how far they will progress and therefore want to get college degrees, or wrestlers who have come from another sport or style (such as Mongolians coming from their national wrestling style, or amateur Olympic-style wrestlers and rugby players have been fairly common in recent years) will go through this route. Younger rikishi and fighters in the lower divisions tend not to have the kind of build that we associate with sumo wrestlers yet and will often climb up the divisions based on power and technique before bulking up to compete at the highest levels.
As a result Koyama will never have received a shikona (ring name) or had to grow the topknot, and it seems that he either had an injury causing him to use the bike to help rehab, or he realised his sumo career was unlikely to progress and decided to change lifestyle to become more traditionally healthy, so it's a bit of a misnomer to say he came 'from' sumo wrestling; he's no more a convert from sumo wrestling - probably less in fact - than Remco Evenepoel is a convert from soccer.
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