The docs aren't wrong; they just make a few assumptions, which are mostly true for existing implementations:

- Your SSB client downloads feeds you follow, plus feeds they follow, and possibly feeds they follow too

- When downloading lots of new feeds at once (for example, the first time you sync) your client makes no attempt to prioritise which feeds it downloads first

- Your client downloads all blobs (attachments) greedily

- Your client indexes downloaded messages (for example, to find the recent messages from all feeds); its implementation of indexing is slow and heavy; until indexing is complete, the app is unresponsive

None of these assumptions is necessarily true of an SSB client, but they're generally true of popular existing clients.