The ITA data are compatible with standard Italian as testified by a Google search: nello stesso tempo has 1,5 million occurrences while allo stesso tempo appears 13,3 million times.
the same search on the CORIS corpus (Rossini Favretti 2000) finds 1771 occurrences for nello stesso tempo and just 1908 for allo stesso tempo, so it is necessary to be careful in drawing conclusions from rough data.