We could say, they are in time; but whilst in time, they do not change; they have constancy; this I think, is the usual explanation.
But consider, would we get the same theory, if we supposed these atoms were not subject to time?
As far as I have learned from the origins of the concept of time, motion makes time.
So if there were only a few stationary atoms in empty space there would be no time, and the atoms would not be "subject to time". On the other hand, if the atoms were moving they would create time and be subject to time.
Also, if there were a few stationary atoms but something else was moving, time would exist and the stationary atoms would again be subject to time.
Note further to down-vote
The link to motion can be observed in that at high speed all mechanisms are slowed down due to their constituent sub-atomic particles being inhibited by the light-speed limit. So time is not universal, (although the moment is). At the speed of light time stops, i.e. there is no time. All this is incidental to my post, which about time in a more fundamental, definitive sense.
That is to say, while a mechanism moving at the speed of light (if it could) would be totally pressed against the light-speed limit and would be frozen "in time", i.e. no motion, elsewhere in the universe someone could be observing so the mechanism would be in time. However in the hypothecated heat-death of the universe there would be only radiation and therefore no time.
Returning to the OP's question:
would we get the same theory, if we supposed these atoms were not
subject to time?
If the atoms were stationary in an empty universe there would be no motion, no time, and the atoms would not be subject to time. However if there is discernable relative motion there is time and they are "subject to time", (taking this expression on face value).
This demonstrates that the OP's question is incoherent, misunderstanding time. Despite the Lucretian atoms not changing, if something is moving they are in time.