That is not true. I have just checked frequencies from Underhill et al. (2009) these are results: Netherlands 40% of R1a is M458, South Sweden 11%, Denmark 8%, Germany 35%, Austria 20%, and Poland 55%. Look that it is only M458 and Z280 was not tested. I would rather tell that R1a in continental Germanics is in majority of Slavic origin, and especially in Germany and Netherlands in vast majority, while in Norwegians and English men it is absent or very rare.
23andMe is supposed to be accurate to modern borders right? So it shouldn't show ancient slavic. If I have Eastern European Ancestry that means the last 500 years, correct?