I would strongly assume that them claiming 100% RES means they have bought so-called Guarantees of Origin (GoOs) [1]. You can only sell GoOs to the extent that you actually produce energy, so while the actual energy that eons customers receive could come from any given source, it is matched by production from RES (which in turn goes to customers who may or may not have an energy provider that provides renewable energy).

[1] https://en.wikipedia.org/wiki/Guarantee_of_origin#Production...