In the same mantra, the question is raised: "How will a jnAni behave?".
And the answer comes; "Howsoever he may behave, he is just such, he is a jnAnai".
In other words he is not regimented by any shAstra or regulation. For such a person , where is the need for the rule that he should adopt the fourth Ashrama among the four Ashramas?
So we should not interpret "AtmAnaM viditvA" to say "after learning by experience" but should interpret it as "understanding by the intellect".It is clear therefore "He who confirms by his intellectual understanding that what he has heard and learnt from the advaita-shAstras is true, now throws away all his desires and becomes a sAnnyAsi" is what is said here.