It is actually simple - I assume it is riding on the same tech platform with configurable FX spread.
Since FSM is technically can be implemented as "wallet" it makes sense that the rate is configurable close to Instarem "wallet". While there is another spread is applicable to direct card deduction.
In gist, it should be done this way but of course the details are shielded from public.
However , it is kinda clear that FSM is not that interested to be card player hence they just outsource this piece to Nium