Tree roots in sewer line?

I'm having a clogged main sewer line repaired, and was told that the roots that caused the problem could come back again after a while. There are no large trees anywhere in the area, so these must be old roots? And if so, can they really "come back" into the pipe at some point? I am dealing with a very reputable plumbing company, and I feel comfortable with them and the work they've done in the past, but I really wonder about roots from a no-longer-existing tree being able to "grow back" into the pipe.
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