First, it is a tiny continental fragment, not a whole continent. Second, their evidence is based only tiny crystals washed ashore, as all island is covered in more recent lava. I will trust this more when they actually drill through that one or two km of lava to recover the actual ancient continental crust.
No. Even the summary does not suggest that Mauritius is an "old" continental fragment ("Mauritius is an island, and there is no rock older than 9 million years old on the island."). They are reporting on zircon inclusions found within lavas and hypothesized to have come from subducted continental crust, rather than subducted oceanic plate (much younger) or the mantle itself.