If that's the case then I believe the law requires that. But if the polling place was supposed to be closed at a specific time and they were allowing people to arrive after that and still vote then that's illegal.
Under Clark County / Nevada polling rules, the polling station can remain open past closing time to process voters who are in line at closing time.
What team Trump is alleging is that they let many more join that line after closing. These are the ballots they are suing to not have co-mingled with valid ballots.