It's being reported that they used ground-penetrating radar. I don't believe any sort of exhumation has been performed - or if so, it hasn't been reported. I think it's likely therefore that this is an approximation/estimate from whatever data is coming up in the radar.

Would be interesting to know many more details: was this completely unmarked? What were the conditions of the burials? How old are they (both in terms of age of remains but age at death as well)?