man Slavs have been non-entity before about 5th century AD
They were a 'non-entity' in Central Europe according to one theory (allochtonic one). But even if they arrived and settled on the territory of the present-day Poland and Eastern Germany in that famous 6th century (500 - 600 AD), they must have come from somewhere. And they would have lived there and speak some language and thus they would have came along with this language which may be called the Proto-Slavic language today. And it is exactly this ancient language which the authors of the paper claim to be the language of that ancient Philistine inscription dating from the 12th century BC..
If that were true, it would be totally sensational from the Slavic point of view. The text could be the first ever known script of the Proto-Slavic language, but of course, more texts and among them longer ones is needed to confirm this intriguing discovery.