The membership numbers is certainly overblown. SNP membership at the 2014 referendum was 25000. It rose rapidly from then to a peak of 125000 in 2019 and is now at 72000. To put that into context the latest House of Commons Library figures are:-
Lab 432000 0.9% of electorate
Con 172000 0.4% of electorate
SNP 72000 1.8% of electorate. (16th Mar)
Labour has fallen from 523000 since Starmer took over.