Actual cycling utilizes core muscles but it isn't all that great at actually building core muscles. Your core is constantly working to stabilize your body, but being in your seat is offloading a lot of work your core would otherwise be doing. It's like the difference between doing planks on the tips of your toes compared to doing planks on your knees. You'll improve your core by cycling since your core is constantly stabilizing your upper body as you turn, pedal, and lean into your bars, but it isn't working your core nearly as much as legit exercises that work your core (planks etc).

Some things will work the hell out of your core, like sprinting with your hands in the drops, since you'll be out of your seat and your core will be the thing stabilizing your body as you hold that horizontal position and resisting torsion between your hips and shoulders. You'll rely on your core to stabilize your body and keep your hips parallel to your seat if you are aggressively climbing with your hands on the flats of your bars and putting gobs of power down into the pedals. The catch is that if you're cycling in a manner that meaningfully works your core, you'd run out of steam before you meaningfully work your core, and you'd be working your legs a lot harder than your core so you'd eventually need to do core exercises outside of cycling just to have enough core muscles to keep up with your legs and the power you're putting down.