That was my point B&Y, those exercises are probably not the best you could utilise, BUT since you don't need to be overly strong, they would suffice in building enough core strength for cycling. Additionally, re your comment that most peoples cores are already strong enough, in actuality, this may be true if you use the strictest definition of "most", however the number of people who do not have adequate core strength due to their lifestyle and work habits is substantial. Also chances are you wont be diagnosed with a weakness in your core muscles until you suffer from an injury related to such issue, so rather than wait to find out, you would do well to include some core training as a means of injury prevention. It takes very little time, requires no equipment and has definite benefits, even if those are just injury prevention.
Furthermore, leg raises are not pointless, they strengthen the lower abdominals. They would be well complimented with reverse leg raises when in the Plank position to strengthen the lower back and also in the side Plank position to strengthen the hip flexors. You could also replace the leg raises with a similar exercise where you lay on your back, knees bend, palms on the ground by your sides, you then lift your arse up, sort of like a reverse plank (maybe this is even the name) then take one leg and extend it, hold in this position, or do reps of lifting that leg up and down about a foot. Press-ups, well they are and they aren't (pointless that is), the prone position, if held properly is going to be working the core, but probably not the best exercise and definitely not the most specific but it would probably suffice.