Just like on the HGVC side, if purchased between January 14th - April 3rd, they'll be able to opt-in. If on/after April 4th, then they will be in HGV Max. If prior to January 14th, there's no firm direction. It appears they are working a way to pay a fee to join HGV Max. $7K was previously mentioned (at least on the HGVC side), but I don't know if that is current/accurate.
If you check in DeX, you'll see some HGVC inventory. We used DeX to exchange into The Grand Islander by HGVC in April. While HGVC inventory appears in DeX, it's very limited at the moment.
ETA: I'm still of the opinion that it's best to purchase a resale unit in the other Club if you already own in one. Sure, you don't get certain benefits, but the buy-in cost outweighs them in my opinion. I'm in the same boat as you. If I decide I really want access to HGVC resorts, I'll buy a resale unit. I doubt that I will so I will be at the mercy of DeX inventory to get to HGVC units.
I think it's because they are using THE Club (and HGVC) inventory for HGV Max. Since deeded owners aren't in THE Club, it's not possible to participate.