Well, we lack context yet. e.g., how often can Imu utilize such an attack, if it's an ancient weapon? And, how much does it contribute to his individual power? Ancient powers' primary use case is large AoE/DC. How much would it help in a 1 on 1 fight? Just like the battle ship pluton or summoning Sea Kings wouldn't be that useful in a 1 on 1.
TBH, this wasn't a traditional powerscaling feat analysis. It was more about Imu's portrayal. Their very first flex dwarfs all other feats in the story.
And note how Imu himself tears apart the posters of both Luffy and Blackbeard, as he is pissed at them. Looks like he cannot "just eliminate" them, even though being pissed and not wanting Blackbeard to proceed.
We lack context to evaluate this. It may be that Imu is simply annoyed with them, but they haven't yet merited extermination. It could also be plot. If Imu erased Luffy/Blackbeard, the One Piece story will end there.
IMO because Luffy and Blackbeard need to survive for their fight, this isn't a good argument against God Tier Imu.
- Roger vs. Whitebeard: We know it was a friendly clash. Later, Whitebeard simply by raising his voice, almost sunk the island.
Imu's attack created a sea quake and tsunami though.
And besides, the Rocks vs Roger and Garp fight didn't destroy God's Valley, while Imu presumably did.
- Enel obliterated an island. He used the maxim to generate thunder clouds which aren't naturally present in Sky islands, however, an awakened lightning fruit could do it on its own
Indeed, Enel is the only other character to destroy an island, but weber saw from his attempt at Skypiea, it takes him a while to do so.
- Issho could just drop multiple meteorites to obliterate the island
Issho could also eventually obliterate an island, but not instantly as Imu just did.
But regardless, Imu still has the unparalleled feat in the story.