OK, I looked at the website and saw a couple of things. First, if there is no train ONLY (no buses as part of ticket) option then I think they have never allowed bikes (boxed or not) on buses. So you have to think the way it was a few years ago before roll-on bikes.
Second, after speaking with Amtrak Customer Service, the "bikes available" on the booking page means ROLL-ON bikes, not check baggage bikes. I little confusing but I am pretty sure that is what it means. If you go to ttps://www.amtrak.com/bring-your-bicycle-onboard and review the "Boxed Bike in Checked Baggage" section, you will see Cardinal does accept bikes as checked baggage. That means you have to start and end your trip at stations with "Checked Baggage" available. I called and confirmed you could do a BOXED bike on the Cardinal even if no slots remain on the ticketing.
Thirdly, contrary to what the Schedule document shows, in January, the Cardinal will operate on Tuesday, Thursday, and Saturday only. When I tried to book a train on Jan 14 (Tuesday), I did get a train with checked baggage but no roll-on.
Finally, like you, I was bugged how no bikes were allowed as roll-on at least thru July so I called Amtrak Website Support and they said yes the slots were being blocked not just unavailable. After putting me on hold, he finally came back and said that due to the weekend, they could not find out why it was blocked but the issue was escalated. He said to check back on Wednesday if it was not open on the website by then. He also confirmed you could box a bike and take it to Cincinnati.
Hope this helps somewhat.
Tailwinds, John