Nammalwar is not saying his soul and Vishnu are one and the same due to Advaitan experience. By "true knowledge", he means understanding the relationship between him and Lord. Nammalwar is an attribute of a Lord and he says everything which belongs to Nammalwar is Lord's belongings. He is feeling sorry that he cannot serve Lord like the Nitya Suris do in the Paramapadam (the eternal abode of Vishnu).
The Pasuram is
yAnE ennai aRiyagilAdhE
yAnE enRanadhE enRirundhEn
yAnE nI en udaimaiyum nIyE
vAnE Eththum em vAnavar ERE
Simple translation based on 12000 based on vAdhi kEsari azhagiya maNavALa jIyar‘s 12000 padi.
Due to I myself not having true knowledge in the matters relating to me, considering myself as independent and considering everything but myself as my belongings, engaged in ahankAram (considering myself as independent) and mamakAram (considering the belongings of bhagavAn as mine) and merely existed (without manifesting AthmA‘s eternal nature); Oh one who resides in paramapadham manifesting the pride of lordship, being praised in all of paramapadham (where true knowledge is practiced without this ignorance) by those residents of the paramapadham praising (your relationship with them)! Once this true knowledge is understood, I realise that “I” can be said as “You”, due to me being prakAra (attribute) of you which is inseparable and in the same manner being your attributes, my belongings can also be called as “You”.
The webpage also cites Nampillai's and Manavala Jeeyar's commentary to the pasuram to understand what Nammalwar meant by him the same as Lord Vishnu.