The security guards will tell you to dismount during high traffic hours. That information is on the FAQ https://www.chicago.gov/city/en/sites/chicagoriverwalk/home/faq.html
The alderman a few years ago tried to pass legislation banning bikes on the riverwalk, but that legislation failed. He also posted the signs seen at the entrances, but those signs are not legally binding as the riverwalk is a designated off-street bike route by the city and are marked as such on all official Chicago Bike maps.  There are also additional yellow signs posted along the riverwalk in which the FAQ refers to, one is posted at the Marketplace section that says "Yield to pedestrians, please dismount.  seen in this video at 11:55 https://youtu.be/bFSKev6vCGI?si=Dr_WyUQVhQTkRS10