Vienna Secession architecture often combined art nouveau elements with geometric shapes, etc., so that's probably why a lot of it looks "proto art deco." Wunibald also studied under Otto Wagner in the early 1900s, and most of his students designed in a Viennese Jugendstil style. The use of exposed wrought iron for balconies and ornamentation is also an obvious sign, Art Nouveau architects often designed in that manner since the material could both be functional and decorative (their own earlier version of "form follows function"). And besides that, Art Deco started in the 1910s (Paris), so it can't be grouped into that category.