They may have been among the best, but I don't think they were outliers. These singers were trained in old traditions of vocal pedagogy still maintained in the 19th century, before singing was compartmentalized into "fachs" and when there was no such thing as a "Wagnerian soprano" or a "Verdi soprano." Yeah, there were lighter and heavier voices, but basically you sang anything you had the power and endurance for, and you were expected to have the technique to do it. Even Nellie Melba seems to have thought that if Lehmann could sing the Queen of the Night, Melba could sing Brunnhilde (or maybe she just thought that Melba could do anything). Of course she quickly discovered that she was wrong, but the idea that she had a "fach" obviously never occurred to her. It wasn't only the sopranos, either. Contralto extraordinaire Ernestine Schumann-Heink, who was the first Klytemnestra in Strauss's Elektra and sang everything from Donizetti to Wagner, had coloratura technique to burn, and so did Caruso, though his repertoire rarely called for it; his cadenza at the end of "La donna e mobile" is unsurpassed, and he shows us a perfect trill in "Ombra mai fu," recorded when his voice had darkened into that of a dramatic tenor who would have taken on Otello and even Wagner.
I'm still discovering singers from the early days of recording whose vocal prowess amazes me. It seems to me that World War II was a cataclysm that amounted to a great dividing line in the history of mankind, and that the history of musical performance practice, including singing, also divides into pre- and post-war. Maybe it's just a personal perspective, or a function of my age (early baby boomer, who grew up with 78rpm records of Galli-Curci and other "golden age" singers). All I'm certain of is that when I want to hear Verdi or Wagner sung as well as it can be sung - from the standpoint of technique and, usually, style as well - it's to singers active before WW II that I must usually turn.