The timetable on the VEX website does state in a footnote that this new route, operating from 30 March, is "operated by SN Brussels Airlines". So it looks like the old Sabena 146s that used to operate into LCY some years ago are coming back, on a surprisingly similar 5 times daily operation. Strange as BA are starting up ops on the same date from LCY to Paris and Frankfurt, and later to Glasgow, and SN Brussels already has extensive codeshares with BA.
Also strange is that there is nothing about this new service on the LCY website.