Yes it is an uninsured moped, on three counts.
1. it is throttle powered and being used as so.
2. One is achieving well over the legal pedelec speed and twice the allowance.
3. The 48v BBS02 will be 500 or 750w rated and marked as so.
You will find the first hurdle will be presenting it to a MSVA testing station and passing the set criteria, the second hurdle will be third party fire and theft insurance and finding someone who can insure it as they certainly will have little or no clue.
You will fine most riders will simply ride them and hope not to be nabbed, if you do get nabbed one might get a way with just the bike being confiscated otherwise you may get prosecuted. As far as the forum goes there have been no cases we are aware of, except one high profile case in Dalston where the guy was prosecuted for death by dangerous riding and was found not guilty.
He wasn't prosecuted for riding an uninsured moped.