The subtlety of possible objects of concentration ends only at the undefinable.
Having such subtle objects extends all the way up to unmanifest prakriti.
Prakriti means the most subtle matter that there is – Satchidananda describes it as:
the primordial basic substance in its unmanifested condition
Patanjali is saying here that the mind has the power to contemplate the most subtle matter that there is, from the gross through the subtle levels.
In this state one may experience bodilessness, or become merged in nature. This may lead to isolation or to a state of loneliness.
Some who have attained higher levels (videhas) or know unmanifest nature (prakritilayas), are drawn into birth in this world by their remaining latent impressions of ignorance, and more naturally come to these states of samadhi.
People belonging to the category of videha and prakṛtilaya are born with the capacity to attain the highest level of samadhi.
The ‘category of videha‘ mentioned in the third translation above refers to someone who has transcended the body (which could also include the mind). ‘Prakṛtilaya’ means absorbed in ‘prakriti‘ – the nature of intelligence by which the universe exists, the ‘primal motive force‘ in the Bhagavad Gita. Before creation, this nature, this prakriti, was the perfect balance of Sattva (light, bliss, goodness), Rajas (passion, motion) and Tamas (inertia, darkness)(the 3 gunas, the primary qualities of nature). A disturbance in this equilibrium gave rise to the physical world.
So this sutra is saying that when one has transcended the physical body, and when one is absorbed in the primary qualities of nature, one has the capacity to reach the highest level of samadhi (meditative consciousness).
Iyengar seems to go further by saying that ‘this may lead to isolation or loneliness’, which the other two translations do no mention. Loneliness has many negative connotations, I wonder whether ‘solitude’ is more what is meant here, but perhaps this will be elaborated on as we read further through the sutras.