They think that since Law's DF is busted, he doesn't need it. But the fight with Blackbeard showed how far away he is from the top. He couldn't even defeat Big Mom power wise when aided by Kidd, but had to push her down the island while muting her voice. So even if Law got ACoC, it wouldn't suddenly make him unstoppable. Like, you can give Law Yamato level of ACoC and he still wouldn't be Yonko level.
On the other hand, Doflamingo said that Luffy has greater potential than Law due to having CoC, which implies Law doesn't have it. Unless it is hidden and only recognized when unlocked.
Also, Law was wasted already and is out of the race, if he returns, his role will be supportive. Maybe going as far as revoking Imu's immortality. It would be odd at this stage of the story for him to reappear and unlock (A)CoC. He won't be a major player in terms of fighting top tiers or the WG immortals.