As a general rule
The definition of what is on and off topic on a given SE site is unrelated to what is on and off topic on another SE site.
So as far as the overlap is concerned, I would not concern yourself with it for purpose of determining what is or is not on topic.
I would expect most questions about bicycling would be on topic here. This would include common bicycling problems, competitive bicycling mechanics and regulations, and such.
What really starts to blur the lines of on and off topic (with any sport) is equipment selection. For example, asking "What bike should I get my son who is new to biking?" would not be on topic. But a very similar question of "What qualities should a bike for young riders have?" might be.
The way I tell the difference between the two is what kind of answers does it bring forth? The first would bring something like "I got my son the Atwood 4000, and it was great for him" and that's not the kind of thing we want. The second would bring answers like "For young riders, the general rule of thumb is 1 speed per 3 years of age (so a 15 yo would want a 5 speed), and the handlebars should come to waist level". (For the record, I made up those stats... they might be good, they might be horrible. They're only examples!)
Good questions should serve two purposes: 1) Help the asker solve their problem. 2) Help future readers solve their problems. A question about equipment selection that is answered with an actual product is probably going to be too localized to help, because the answer is going to be specifically geared toward that person, while an answer with formulas, guidelines, rules of thumb, legal information, etc is likely to help a lot of readers in the future. They can just plug their own variables into the formula and rules and get the answers to help guide their selection.
I would assume the other SE site would have the same criteria. But again, this isn't about what is allowed on their site, it's about what is allowed on ours. We wouldn't allow it on ours just because they denied it on theirs.