Quick answer
Yes, offline growth is officially confirmed
The official Roblox description still says Your Explosives Grow Offline. That is enough to answer the core player question, but it is not enough to publish exact offline rates, caps, or breakpoints.
If you are returning to Nuke Your City, check your current explosives before spending on another machine or upgrade. The safest assumption is that offline growth changes your next buy order, even if the game does not publicly expose the exact formula.
If offline growth is the only thing you want to check before logging off or jumping back in, this page gives the focused answer faster than a broad beginner guide.