Imu cut down Teach's previous first bounty so it wasn't even 3.9 billion or even 2.2 billion in this photo, that old photo is different than this one:
One of the arguments that ''they didn't know Dragon was a D.'' , thats absolutely wrong as even Sengoku and Akainu knew Dragon's background before during the Marineford battle and they later announced it to the whole world.
''The flesh and blood son of the revolutionary Dragon!''
Garp also told Aokiji during the flashback that his son is Dragon and he become a revolutionary, and Ivankov said 2 years ago everyone learned Dragon's background after Marineford battle:

''Looks like the whole world knows your background now Dragon.''
So the excuse can't be they don't know Dragon is a D.
We also know obviously Imu knows more about D. than we know:
''D. is the moniker of our ancient enemy.''
Another thing is, what scares Imu more looks like finding One Piece rather than the Revolutionaries:
''Without that blunder there would be no pirates chasing the poneglyphs in pursuit of treasure...''
There are 2 possible explanations:
1- Imu knew about Luffy is Nika (Sun God) and its direct counterpart (Moon God or Darkness) whatever thats is Teach, and together or separately, only they have the power to overthrow him. Similar to their predecessors Roger and Xebec. So Dragon is not one of those 2, thats why Imu is not worried about him.
2- Since Imu is more concerned about poneglyphs and pirates chasing One Piece treasure, and Dragon is not a pirate, even when he is the ''most wanted man / world's worst criminal'' and also a D. , Dragon is still not as dangerous as a D. pirate being close to finding the treasure.