Forcing guests to wait until Borchu to sit down in unclaimed seats is unreasonable. That is way late, and by time the minyan is up to Borchu guests aren’t going to suddenly start moving around into unclaimed seats in midst of davening. If Baruch Sheomer passed and a seat is unclaimed, it is late enough to be hefker.
Also, often when a guest asks others sitting near an unclaimed seat whether it is available, they either don’t know, shrug or they say it is late enough that it is available. Then the late bird shows up thereafter when the guest is already seated there.