A PLO spot in high roller WSOP

Got into a bit of a debate with somebody about this PLO spot at 2:48:50 — https://www.youtube.com/live/U5HEdzSOjvc...

Chidwick (SB) vs Gryko (BB). Final table, 4-handed. 250k/500k-500k level. 28 BBs effective (Chidwick). Gryko is close to chip leader with 36 bigs. There’s a shorty at the table with 13 bigs.

Chidwick: AsKc6h6c
Gryko: KhTc9s5s

Pre: Chidwick limps, Gyko checks back

Post: Chidwick leads 1/3 pot, 30% pot (and gets called by Gryko twice), then checks river.

Runout was: Qs8sJs - 4h - 6d

When checked to, Gryko bets his 9-high flush for half pot (2.1M into 4M) when remaining effective was 12M.

The discussion I had had about this was based around the question - what would a solver/GTO do?

Curious if anybody here thinks Gryko’s river bet was definitely egregious, or if it’s very likely solver-approved? First just chipEV, but also how does ICM factor info it? In general, how many flushes do you think we should we be able to bet after being led into twice in a configuration/board like this?

Also, do we think Chidwick is balanced here or likely over bluffing? If he did have the ace-high flush (or a straight flush), does it actually make sense to go for a checkraise?