Don't compare Imu's Arrival to those Villains,
He appeared Solo immediately after SHs & Allies were Celebrating their Victory, (Against Sommers + Killingham + Gunko)
Which means He should be similar to One of these:
1. Kuzan showing up after Foxy Games (Foxy + Hamburg + Porche)
2. Kuma showing up after Thriller Bark Nightmare (Moria + Absalom + Perona)
3. Kizaru showing up after Auction House Incident (Three Celestial Dragons)
None of these were Final Villains, They only appeared to give entire SH Crew a "Reality Check",
What's common here:
1. Entire Crew were Overwhelmed & had no way of harming their Opponent
2. Enemy threatened to Kill a SH (Kuzan with Robin, Kuma with Luffy & Kizaru with Zoro)
3. Enemy eventually decided to spare Luffy for different reasons
4. There is a Legendary Old Man behind such Enemy
(Kizaru was sent by Sengoku, Kuma mentioned He was modified by Vegapunk, and Kuzan teased his Mentor Garp)
Well, there is a problem in your logic. This is a fair point, but we are so far in the story that the villain who can overwhelm the MC crew shouldn't exist. Why? Because there will be no timeskip. So if some villain has this overwhelming power, this means he will be defeated by the MC in the future. In another case, readers wouldn't be able to say that MC was able to break through any of his obstacles. For example, Luffy didn't defeat Big Mom, but it was obvious that she was weaker than Kaido. On the other hand, Imu, the
King of the World, is the only man who has a title like this. If someone like Teach will defeat him in a weakened state, this wouldn't mean that Luffy was able to defeat Imu, or that Teach was able to defeat Imu fair and square.
Now what does this mean?
If Imu is not an FV, he will struggle in the battle against Luffy and Loki, wouldn't be able to do much, and then he will be absorbed by BB. In Naruto, when Madara was fighting against buffed Naruto and Sasuke, we saw how we started to struggle against them in battle. And then Kaguya appeared and took the spot of the Final Villain. But no one was saying that Madara is stronger than Kaguya.
If Imu is the Final Villain, his power will be overwhelmed, and he will destroy Luffy and Loki. So even if he is weakened after battle, because he basically risked his life and was absorbed by BB, you will still be able to say that after BB's death, he will be revived and play the role of the Final Villain. Also, BB could just lose to Imu and never be able to absorb him.