Refer to the place in the linked article where the part I quoted appears in a section called "Limitations"—the bookmarklet is stopped in its tracks when it encounters style sheets that are loaded from off-site (but that are still loaded nonetheless and have an effect on the current page). Considering that the user is the one who invokes bookmarklets and has their own personal accessibility reasons for doing so, that's an unwelcome limitation.
There's a similar effect with CSP: server operators and content authors can use CSP to prevent the use of bookmarklets that ordinarily work by overriding the author's style rules through $element$.style (i.e. the style attribute), inserting style elements with the user's own CSS, or loading/linking against off-site style sheets.
See also:
Google's WEI ("Web Environment Integrity") proposal from earlier last year that was justifiably opposed by pretty much everyone[1]
The W3C's Priority of Constituencies[2] (part of its Web Platform Design Principles) and its Ethical Web Principles related to putting users in control[3]
