Interesting read but that part really made me question my own sanity. It’s probably just lost in translation.
It is not wrong that you can have a file (bits and bytes encoded in the shape described by a file format) on some remote point. If you have an index of those files where you can programmatically choose between multiple files that could even pass as a crude "file system". But I doubt this is what they meant to refer to.
It is likelier they wrongly assumed a file system is the system in which a file is organized, where in fact they meant file format.
A filesystem stored in a monolithic file is not so constrained. I work with Harmonix games as a modder, and they use a bespoke format called "ARK", which is a two-part format. There exists one or more "ARK parts", which in implementation are virtually concatenated, and then the HDR lists a binary offset from the beginning of the first ARK part to tell the games where the individual files are. This could also be called a "container" conceptually. But none of such files are interleaved in any way except for the audio files, which use encrypted multichannel Ogg Vorbis streams plus a bespoke header to aid in seeking.
Plus, the context was explaining it to laypeople, where the specific jargon is less important than imparting knowledge and understanding.
So yeah, many SSDs are just huge, non-solid archives.