The largest Jewish community at that time was in the former lands of the Polish-Lithuanian Community - i.e. Galicia (part of Austria-Hungary) and the Pale of Settlement in the Russian Empire. If you define 'worker' as someone working in a mid to large factory - i.e. classic proletariat, then yes, most Jewish workers were socialists, but most Jews were not workers under that definition. If you define worker as poor, manual labourer or bottom rung tradesman - peddlers etc. - then most Jews were indeed workers, but only a minority were socialists.  

This was at a time when official Socialism took a very hostile attitude toward nationalisms of all kinds

Very misleading because the most popular variant of socialism among poor Jews in the ex Polish-Lithuanian lands at the turn of the century was Bundism, a rather peculiar form of socialist Jewish nationalism. After that you had strong allegiances both to the more or less anti-nationalist Russian SD's as well as Roza Luksemburg and Feliks Dzierzynski's strongly anti-nationalist SDKPiL, and the nationally oriented PPS and its Galician equivalent.