Card says:
"RETURN TO AIUR
Immediately place up to 8 transports on any navigation route(s). If you cannot place all 8 you may first voluntarily destroy any number of your transports.
Your transports cannot be destroyed (except voluntarily) during the
next
round."
So an Overmind player, which plays his level III card
"THE SWARM FLIES
Destroy up to 1 enemy transport on each navigation route."
can destroy Tassadar's transports, because he uses his leadership card in the current round?