No, I'm not confusing anything.

Dropbox and onedrive can handle backblaze zipping through and opening many files. The risk is getting too many gigabytes at once, but that shouldn't happen because backblaze should only open enough for immediate upload. If it does happen it's very easily fixed.

If it overloads nextcloud by hitting too many files too fast, that's a legitimate issue but it's not what OP was worried about.