"They had a previous exclusion of gain on home.a few years ago."

There are two different "2 year" rules. One is for ownership (2 of the past 5) and one is for other sales (not other sales closer than 2 years).

"Won't affect future sale of home where postponed previous gain reduces the basis of home sold?"

The rule for "you have to buy a more expensive home next time" is a couple of decades in the past. It was replaced with the exclusion rules.