In his book "Vaishnavism Its Philosophy, Theology and Religious Discipline" SM Srinivasa Chari (Motilal Banarsidass, 4th reprint, Delhi 2018) writes on page 148:
In the light of these statements the stray sentence appearing in the Svetasvatara Upanishad that Siva alone existed is to be interpreted in the sense that Siva is Narayana.
random, chance, freak, accidental are some synonyms of "stray".
Scripture talks about prakshipta(interpolated) and Tamasic scriptures as scriptures that can be discarded -
but Does scripture talk about "stray sentences" in other scriptures?
EDIT: added clarification.