There is a separate Siberian admixture on K15, so I don't think that Eastern_Euro contains Siberian.
Mari people score both Siberian and Eastern_Euro - so Eastern_Euro represents the Non-Siberian part of their ancestry, while the Siberian part is represented by Siberian component.
I have GEDmatch kits of around 300 modern ethnic Russians, and all of them score some Siberian on K13 (and on K15 most probably as well, though I didn't check all of them on K15 yet), but Early Slavic samples do not score any Siberian. So Siberian admixture in modern Russians is of Non-Slavic origin.
Interestingly, these Early Slavic samples (Markowice7, Niemcza18, RISE568) also don't score any East_Med on K15. Some of modern Poles don't score East_Med as well (for example father of mlukas doesn't get any East_Med on K15), but the majority do.