Yes - someone can tag in... or NOT... and the filibuster can continue.
Why do I say “not?” Because in recent years, filibusters have been entirely notional - a simple threat to continue talking. If the Senate doesn’t think it has the vote for cloture, they simply move on to other business and no real filibuster takes place.
This is a bold move by Rand as, typically, the senator who has the floor is afforded the opportunity to continue speaking before the motion for cloture is made. If there is a general belief that the cloture votes WON’T be made, then another senator COULD tag in.... or NOT... and the senate moves on to other business.
If there is a belief cloture will be made, it will come when Rand exhausts himself.
Thank you for clarifying this.