At one time all Oxxford coats were shipped with buttons attached to sleeves. I think that it might have been any additional show of handwork, as the buttons sat like fat soldiers in formation atop thick shanks.
The garment that you came across was probably shipped that way. If the buttons are sewn flat to the sleeve fabric, then it is possible the garment has been altered.
Creating functional sleeve buttonholes is not an issue with Oxxford garments.