I think Imu really just acts in person when the truly "forbidden" things are getting leaked.
Ohara did some research and stuff, but I think they still didn't learn about Imu still ruling the world or anything like that. Just about an ancient kingdom once existing and stuff like that, but all rather vague.
That's why a Buster Call as a warning to all the other nations was good enough for Imu and actually a smart move to "show" them the power of the WG and what happens if you ask the wrong questions ("devils of Ohara" propaganda).
In terms of Sabo on Lulucia: Dude was about to drop a THE most forbidden/bombshell info in the world with all the marines and God knows who else wire-tapping because of this mickey-mouse phone-line Sabo had to use.
That's why Imu just went scorched earth on Lulucia and Sabo.
Imu didn't want to send out warnings or show the WG's power, they just wanted to erase any trace of the kingdom/Sabo/this conversation point blank.
That's how you can explain it. Easier explanation would be: It's just kind of a plothole bc Oda didn't plan this much in advance back at Ohara.