Here is Jordanes' text in English translation:
Within these rivers lies Dacia, encircled by the lofty Alps as by a crown. Near their left ridge, which inclines toward the north, and beginning at the source of the Vistula, the populous race of the Venethi dwell, occupying a great expanse of land. Though their names now vary amid various clans and places, yet they are chiefly called Sclaweni and Antes.
The abode of the Sclaweni extends from the city of Noviodunum {New Town, modern Isaktscha, Romania} and the lake called Mursianus to the Dniestr, and northward as far as the Vistula. They have swamps and forests for their cities.
The Antes, who are the bravest of these peoples dwelling around the bend of the Black Sea, spread from the Dniestr to the Dniepr, rivers that are many days journey apart.
harbornet.com/folks/theedrich/Goths/Goths1.htm
Yet, ancient chroniclers name people living in Central and Eastern Europe befor 550 AD
Very true, Herodotus writing in the Fifth Century BC mentions several peoples in the area (including a race of seasonal werewolves) and Tacitus wrote of the Venedi at the end of the First Century, but the text above from 550 AD is the oldest one to use the name Sclaweni.
Such an argument is irrelevant. According to many specialists, those names are of Indoeuropean origin.
The names of Polish rivers meaning nothing in Slavic languages is not irrelevant, even if they are of Indo-European origin. Names from different branches of the Indo-European language family would be consistent with the widely accepted theory that posits Indo-Europeans migrating in waves, with the Slavs being the last wave.
If they are so old, we may say they may have been formed in a language which is now called the Proto-indoeuropean language.
One may say this, but although Poland has been proposed as the Proto-Indo-European
Urheimat, the most popular candidate for this place is the Pontic-Caspian steppe. Others argue forcefully for Anatolia.