I guess you didn't pay enough attention to that discussion within the group. R1A-M458 isn't German, or Germano-Slavic. Arguably, at over four-thousand years old, he's not even proto-Slavic. Although the Romans undoubtedly called his descendants Slavs. In your case, R-L1029(estimated age 2 to 3 thousand years according to YFull) is considered to be "a Slavic Marker" as its population dispersal is consistent with the height of the reach/expansion of the Slavic population across Europe. But the operative part there is "at the height" as things collapsed from there. Nobody in that Facebook group would argue that your R-YP445 Ancestor(estimated age 1300 to 1800 years old; or roughly 200CE to 700CE) may have identified as either an Anglo or a Saxon. Or he might have identified as a Slav himself, even if his grand-children didn't.
Going by the wikipedia entry on Anglo-Saxons (would provide the link, but as I'm newly registered, I can't):
Odd how history seems to indicate there were Slavs running around in Germany in the appropriate time frame for even R-YP445 to be a Slav. Of course, they then integrated into the local population, and their cultural identity shifted over time from being Slavic to being Germanic. Just from the quote above, I'd say R-YP445 was "probably a Slav" but that doesn't exclude his descendants from being Germans, or "Anglo-Saxons" from Britain.
I personally identify as an American who happens to be descended from R-L260 myself. Which is how I'm aware of that discussion. While my descendants can likewise make the claim to being "of American Descent" that does not mean that somehow magically turns R-L260, R-M458, or R-M459 himself into an American as well.
If you reviewed that discussion, I think you'd find the same admin who was hyper-critical of M458 being called Germano-Slavic also had bad things to say about calling M458 Slavic as well, because M458 probably had never even heard of a Slav(considering it's what the Romans called them--Slaves; however, that wasn't until a couple thousand years after he was dead and gone). But people are lazy, and as the vast majority of M458's descendants happened to live in the area that became known as Slavic, thus M458 "became a Slav" retroactively.
The Slavic identifier, historically inaccurate as it is, at least is defensible, as M458 is both likely to have originated there, and M458 populations remain there in numbers to this day. Particularly given L1029's expansion, it's safe to say that M458's descendants probably had at least some influence in the form Slavic society ultimately took.
The claim for M458 being "Germanic" however? That has a number of problems, and only has a leg to stand on if you define it using Prussia as your guide, and even then, there are problems to be had.