The Self is not a Witness (saksin) in itself; it is a Witness only in relation to the (illusory objects that come before its light to be) witnessed. Being nothing but the inmost vision, it is not accessible to thought or speech.... Even though this inmost Self is thus immediately knowable, because it is self-luminous, and though it is the Witness of all metaphysical Ignorance and its effects, nevertheless it remains unknown until the advent of some definite increment in knowledge has been sought and obtained. Therefore it is only from hearing Vedic texts like 'That thou art', and not from any other source ( such as obedience to injunctions or halting of impressions) that unshakable knowledge of reality as the one Self results. (Sambhanda Vartika of Sureshvara)(from The Method of the Vedanta by Swami Satchidanandendra pg. 205)
This excerpt is from Chapter IV: Pre-Sankara Schools. None of those schools are mentioned as such just their general purport. What is being impugned is the idea that following through on the ritual injunctions of the Vedas will bring realisation. Doing so will bring only merit. Likewise the well known aim of Yoga – citta vriti nirodha (elimination of mental impressions) – will not cause realisation. Swami Satchidanandendra writes re the apparent redundancy of tat tvam asi:
Nor is it correct to think that if the Self is self-established, the texts which give knowledge of it are inauthoritative because they only repeat what is already known from some other source. For we know from experience that things that are at first not known become known only when some definite increment in knowledge has been sought and obtained , and what is a matter of common experience cannot be questioned.
The Vedas are a pramana or valid means of knowledge and for the Vedantin add surety to what might be construed as mere metaphysical speculation. The message is simple – only knowledge banishes ignorance.