Why is this considered a spin-off yet...
... Toyko Drift is included in the numbered series (although not titled as such)?
Surely Toyko Drift is just as much, if not more, of a spin-off - it's only connection is Dom for about 5 seconds at the end and a retconning of Han into the series.
Hobbs & Shaw are both characters with way more screen time in previous numbered F&F films and therefore this film is much less of a spin-off.
Really though they should just drop the numbers altogether and just crack on with the series. It's not like the James Bond producers felt the need to keep numbering their films.