So we can forget about that! Probably a Serb putting that into the text.
Just because citation is missing that's no proof that Serbs did not participate in that battle.
If we take a look at Polish army at that time it was mainly recruited from local population as conscripts by "Pospolite Ruszenie" which was the duty of every free man (meaning the land owner) of joining the army at call to arms by the King. The call to arms was performed by "Starosta" who issued a proclamation on behalf of the King in his district and each nobleman, knight and land owner had to join Kings Forces for a period of time at his own expense. He was also obligated to recruit few extra soldiers from his land holdings but the rest of the time they were free to take care of their farms so there was no regular army to speak of. This law only applied to the campaigns within kings' realm. Any service performed outside Kings Realm they had to be compensated by the King, provided with food and in case they were taken hostage it was the Kings responsibility to pay the ransom. Army recruited in this fashion usually consisted in 1/3 of nobleman serving only at the time of need.
Very small contingent that can be called a regular army meaning serving King all the time was mainly consisting of mercenaries from within the kingdom, Poles as well as our southern neighbors, Czechs, Hungarians and Serbs. Besides Polish nobleman and Knights mainly formed Heavy cavalry units. Although Polish knight at that time had a lighter armor then western counterpart because of the conflicts with Tartars he still carried a lance, 120cm sword and 30cm dagger protected by articulated metal armor.
At that time there was no home grown Polish light cavalry units. That what one might think of as light cavalry were the servants of the Knights, equipped with lance, sword, some with bow and some with crossbow protected by leather armor. Bulk of the light cavalry units were from the borderlands of the kingdom as well as from Lithuania. The rest of the light cavalry were from the southern kingdoms, of those the Serbs, Hungarians, Moldavians, Wołochowie (sorry don't know the English term for them) as well as Tartars were particularly valued because of their experience fighting Turkish light cavalry units.
We know that the Serbs served in light cavalry units at that time so it would make perfect sense for them to participate in this battle given that they were at the service of one of the Polish nobleman and all of the Polish Lords participated at that battle, after all it was one of the biggest battles in the medieval Europe. Even Sienkiewicz in Krzyżacy mentions them.
Zgromadzili się wielcy sławnirycerze oraz różne narody. Z Witoldem przyszło czterdzieścilicznych chorągwi, polskich wraz z Mazurami było pięćdziesiąt. ByliTatarzy i dzikie watahy Besarabów, Serbowie i Żmudzini