Everyone can have their opinion thats just mine....I think there are ways for them to track the biometric date, and also people get the "docs recd" email after biometrics but before they actually did send them like Smelton, but it doesn't really matter. its just a matter of everyone surmising...
That's just an auto-email triggered by attending the biometrics appointment... it doesn't mean anything for US applicants and really should not be sent to them.
In some countries, you submit your documents in person at the biometrics appointment, so they get the email to say the documents have been received... so for them, their processing time starts from the biometrics date.
In the US, however, biometrics are taken by US immigration on behalf of UKVI, so no documents are submitted at the appointment. Therefore processing cannot start on the application at that time, because the application documents have not been submitted yet.