The author is assuming a certain context here: that we are interested in the specific problem of transferring binary resources to a web browser. I appreciate in

The author is assuming a certain context here: that we are interested in the specific problem of transferring binary resources to a web browser. I appreciate in isolation this sentence seems incorrect, but if we assume good faith on the part of the author it's clear what they're trying to get at. They are using 'base64 encoded' as a shorthand for 'as an embedded base64 blob inside an HTML page.' - to distinguish it from 'as an individually requested resource'. So what they are referring to is that by embedding resources as base64 blobs inside HTML pages, and transferring those over SSL, an observer sees one large encrypted bundle being requested. If you transfer the resources as separate response entities, then an observer sees the large HTML bundle request, followed by the series of specific resources - and they can infer from the sizes and patterns of those requests some things about the page requested or the resources used. (for example, if I know the size of the HTML and every image on wikipedia, perhaps by observing the set of sizes of resources being downloaded by a client over HTTPS I can determine which wikipedia pages a client is browsing?)