In this type of questionnaires, the statements (called “items” in the technical literature) are not chosen mainly because they describe what usability is but because they are affected by it (i.e. the question is not “how does this affect usability” but “how does usability affect this”) and the ratings are correlated together. For the SUS, they were originally selected from a pool of 50 candidate statements based on a small study with two systems that could have been used frequently by the participants, which might explain why this question was retained.
If a statement does not seem to relate to the usability of the website for the participants, you would expect little variability, no differences between products regarding this particular statement and no correlation with the rest of the questionnaire. Technically speaking, it could limit the theoretical maximum score you could obtain with the questionnaire (i.e. bias) or contribute some random error to the total score. This would in turn slightly limit the power to detect differences but, as Ben noted, it would not bias a comparison between two websites in the same category (previous iteration of your design, competitors…)
It also conceivable that some respondents see through it and answer anyway based on their perception of the website itself, not the task it is designed to support. However, if you have several such irrelevant or awkwardly formulated statements in a questionnaire, the respondents might react negatively, complain about it or become less cooperative in general (I have seen it happen with the SUMI).
If you want to know more about the SUS, the original reference is
- Brooke, J., (1996). SUS: A "quick and dirty" usability scale. In P.W. Jordan, B. Thomas, B.A. Weerdmeester, and A.L. McClelland (Eds.), Usability evaluation in industry (pp. 189-194). London: Taylor and Francis.
I also gave some other interesting references in this answer to another question about the SUS You can also check this question which raised similar issues and even includes an answer from the creator of the SUS.