Correct. You'd need to do some sustained intervals at a decent pace. They don't necessarily need to be on hills but you do need to ride hard for some periods preferably in a big gear. Personally, I don't have any easy access to stairs so I'd rather ride my bike.

We have a local ski hill that everyone climbs for exercise. It's not scenic but it's about 800m of elevation in 1.8km and contains many steps (natural and man-made). I usually do it once or twice a year and can ascend in under 40min without killing myself. Fastest climber and record holder was a local pro cyclist. Top stair climbers look just like top cycling climbers.

That said this event is a tribute, not a race, and I think most of the participants will be firefighters or members of public safety, not skinny, hill climbing cyclists.