Complex sentence: The 1PL assumes not only that guessing is not present (or irrelevant), but that all items are equivalent in terms of discrimination, analogous to a common factor analysis with identical loadings for all items.
Split into simpler sentences: The 1PL assumes not only that guessing is not present (or irrelevant), but that all items are equivalent in terms of discrimination. Here, the trait is analogous to a single factor in factor analysis with identical loadings for all items.