The Invulnerability ultimate ability is keeping their existence safe from the effects of the reversal of time, and it will continue to do so as long as they have divine energy to use it.
Thanks to the Domination ultimate ability of the Crown of Nobility, they will be able to last much longer than they can against the reversal of time. But they still won't be able to last forever in this state.
As long as they can hold on until the god of travelers becomes too tired to run and stops running to the past, everything will be fine. But if they can't endure the consumption of divine energy and run out of it, then they will be in big trouble.
While other high gods lost the struggle and became submerged in the River of Time, Legion was able to stay afloat in the river for some time. Bit eventually, they ran out of divine energy to continue resisting.
