This is not Deretić's blaberings, it is what you will hear from modern Indo-European linguists. Avestani, Greek, Latin and Anglo-Saxon are all Indo-European languages, and Serbs are Indo-Europeans. (Lithuanian is even more related than previous mentioned languages). Avestani and Greek are here as examples of older literary languages who better perserved previous stages which long ago were common for all Indo-Europeans.
Wiktionary quoted University publication. I guess it is not hard to look at source:
J.P. Mallory and D.Q. Adams, "Protect", The Encyclopedia of Indo-European Culture (London: Fitzroy and Dearborn, 1997).
Isti does not exist in Russian, Polish, Ukrainian and Czech. That is first. Second, theare are words derived from same root, and that words ahve similar or same meaning in all Slavic languages.
Ukrainian пасерб is borowed from Polish pasierb, same as Belarussian па́серб. Further more, meaning is stepson, посвојеник, усвојеник, посинак, this sierb could be somehow related to Proto-Slavic sьrbъ, but it is just possibility, there is no similar word in other Slavic languages.