Based on the french translation I read, and on this panel and the one after, I think Imu thought Eris and Teach died. This happens right when Kuma uses his DF on them. So basically, to Imu, their presence was gone right when the explosion happened. Maybe this is still left to interpretation but right after that, Imu tells Rocks "Now then, kill everyone on the island" (so marines included.)
This confirms that Imu had no idea Teach was the son of Xebec until chapter 907, when Shanks/Shamrock came to Mariejoie and told the elders "There's a pirate I need to talk to you about". On this instance, I'll say that I think Oda baited us with Shamrock and it's actually really Shanks and not his brother. Shanks is the only one who could know that since he had a fight to the death with Blackbeard and we know BB loves to talk shit. + We also know that they are from the same island and suffered a similar fate.
So that's Shanks talking about BB being a Davy.
Imu having a meltdown because he realizes the will of both Davy and Joyboy is still alive through Luffy and Blackbeard and both have the powers to go and take control of Shiraoshi (Sea god.) This was the last chapter before Wano so even if Oda made up the Nika fruit on I'm sure he had it in mind at that time.
Probably the moment he tells them Teach & Luffy needs to be erased. TBF with everything in context it still is insane that he didn't go on Elbaph right at this moment, but I guess that's just the price of story telling.
Does it make it worse or better that Imu had no fucking idea who Teach was before Shanks told the elders ?
In any case, Imu downscale cause his observation haki couldn't detect that they got teleported and didn't die.
WHICH MEANS ???? ENEL UPSCALE because his observation haki is better.