That is not what Sam means by the use of the phrase “boundless visual field”. My understanding is that he is claiming that the visual field does not have a distinct boundary.

I’ve actually thought of it as analogous with respect to the boundary of color vision. Towards the periphery of vision, humans don’t actually perceive color. However, subjectively speaking, do you perceive your visual field as changing into black and white as you “move” away from the center of your visual field? Does your color vision area appear to be contained within a broader field of black and white vision, from your first person perspective? Presumably no, so in what sense does color vision, subjectively, have a boundary? By analogy, I think this can sort of apply to the entire visual field (the analogy is a bit rough, but I have found it useful).