Honestly, I'll be surprised if that is indeed the case. Aside from the Fear the Walking Dead webisode tie-in miniseries about the submarine everything Walking Dead related has aired on AMC, and IIRC, the upcoming Daryl and Negan/Maggie spinoffs will be airing on AMC itself, so it would be odd if this Rick/Michonne miniseries were an AMC+ exclusive.
Also, I could be guilty of reading tea leaves here, but the
poster image released for this miniseries has both the AMC and the AMC+ logo on it, which suggests to me it'll likely be available on both. If it were an AMC+ exclusive, I would think they'd just have that logo and not the one for regular AMC. But I could be wrong, and feel free to treat this as the baseless speculation it is.
Now what I could see happening is that each episode would be released on AMC+ a week before it airs on AMC, which is already the case with Walking Dead, Fear, and World Beyond. Or they could even release the entire season on AMC+ at once and air each episode on a weekly basis on AMC, which I've heard is the case for many AMC shows this year. Again, totally baseless speculation on which I could be wrong, but I personally think it's too early to throw in the towel and assume we need to switch to AMC+ in order to take part in the future of the Walking Dead franchise.