According to this answer, Thenkalais believe the liberated souls have the power of creating their own universes, but not the universe that is common to souls trapped in samsara; only Brahman can can create, sustain, and destroy that universe.
The Sri Bhashya of Ramanujacharya supports this:
In the same way as the highest Person creates out of himself, for his own delight, the world of the Fathers and so on; so he sometimes creates such worlds for the enjoyment of the released souls. But sometimes, again, the souls using their own creative will-power themselves create their own worlds, which however are included within the sphere of sport of the highest Person.
So it's clear liberated souls can create their own universes. Now obviously this has to be the case, because the liberated souls get all the siddhis in existence, and if baddhatmas (souls trapped in samsara under maya) like Vishvamitra can create entirely new heavens, the liberated souls should be able to as well.
However, according to the question to that answer, Vadakalais do not believe the liberated souls have any powers of creation. They say:
that the Free and the Freed Souls have no power to create, or, for example, make a kosmos.
So, they can't even make a cosmos according to them, which is clearly in conflict with the Sri Bhashya citation.
How do Vadakalais explain this? Do they mean that the liberated souls don't have any powers of creation, or just that they can't create the universe common to Baddha Jivas? Or is the author of the article cited in the question just wrong?