Infected devices are not owned by the attacker. So, it doesn't matter to the attacker if the bots are found.
Mirai was so successful that if a device was found, the attacker could simply get another.
Other botnets might be carefully controlled and tended by the attacker. They might be difficult or costly to maintain or to add new bots. In these cases, the ...
A good translation is being able to control. It doesn't matter whether you control the system via an exploit and installing a backdoor or having an infection vector that connects the device to a bot-net orchestrator or anything similar.
Once you have full control over the system, you have hijacked it.