Strictly speaking, that's correct. It's not a realistic imitation of a "an
actual make or model of any firearm other than one the appearance of which would tend to identify it as having a design and mechanism of a sort first dating from before the year 1870".
So an M41A Pulse Rifle wouldn't be a RIF, even if the man on the Clapham Omnibus might think that it is. However, I wouldn't fancy taking that on a bus and then arguing the toss with an armed response unit.
Whereas an imitation of an actual gun (which shoots actual caseless ammo)
would be a RIF, even if it looks like someone knocked up a joke Kraut Space Magic gun out of cardboard.
If your followup is "That makes no sense", then you'll just get sad nods from us. It's not a great law.
For bonus giggles, you could argue that this can't be a RIF.
Because:
1) It has a design and mechanism of a sort first dating from before the year 1870, e.g. a
Werder 1869.
and
2) It's a pretend space gun from the future.