I didn't make those claims; linguists and phonologists studying Chinese linguistics and phonetics did. They looked at linguistic and phonetic
shifts of today's dialects and compare it with Old and Middle Chinese. The more shifts, the more recent the dialect. The more conservative, the least shifts, the older the dialect.
While Beijing Mandarin has retained elements of Middle Chinese in its grammar and syntax, in pronunciation it has reduced the 8 tones of Middle Chinese to 4, lost the final plosives (p, t, k), as well the final
-m.
The two language groups with the least number of shifts are the Yue and Min groups. Cantonese has retained the 8 tones and final stop consonants, but has lost the initial voiced consonants (voiced b, d, g) found in the Wu group.[SUP]1
1. Ramsey, S. Robert (1987),
The Languages of China,
Princeton, New Jersey:
Princeton University Press,
ISBN 978-0-691-01468-5.[/SUP]