Doesn't matter that the pax had already boarded, even if the plane was already taxiing or even departed it could have been turned back and pax deplaned per the force majeure and unforeseeable conditions clause of the COC about the possibility of denying service to anyone at any point for that kind reason. This is not only in UA's contract but also AA's, DL's, WN's. So indeed the 4 passengers had already boarded legally but they were also legally requested to deplane as long as this came with the relevant compensation and as long as it's demonstrated that it was because of an unforeseen circumstance.
Force Majeure can not be used for operational movement of employees.
Per definition :
A Force Majeure clause (French for "superior force") is a contract provision that allows a party to suspend or terminate the performance of its obligations when certain circumstances beyond their control arise, making performance inadvisable, commercially impracticable, illegal, or impossible. The provision may state that the contract is temporarily suspended, or that it is terminated if the event of force majeure continues for a prescribed period of time.
A typical list of force majeure events might include war, riots, fire, flood, hurricane, typhoon, earthquake, lightning, explosion, strikes, lockouts, slowdowns, prolonged shortage of energy supplies, and acts of state or governmental action prohibiting or impeding any party from performing its respective obligations under the contract. So if, for example, a hurricane occurred that shut down a port, the seller planning to ship its goods through that port would not be liable for late delivery of the goods.