SG2 and VG10 (like VG1, VG5 etc) are steel names created by Takefu. As Takefu doesn't produce steel, but only laminates or trades them, the steel may be traded by the steel producer under another name. For some steel like VG10 which is produced by Fujikoshi, Takefu seem to have the right for exclusive use in knives. Could be different with SG2 though, so that the statement made by Koki could be correct.
The first has Kobelco Steel R2 versus GSB Acero R2
The second had Takefu SG2 versus SRS-15.
If you compare Kobelco Steel R2 with Takefu SG2, you'll see they're exactly the same.
Besides, if you scroll over R2* beneath the chart at the first link, you'll see it clearly says Kobelco Steel R2 and Takefu SG2 are the same. The difference is makers who use Kobelco Steel R2 tend to forge and heat treat their knives. SG2 knives tend to be stamped.
Under the second link, if you scroll over SG2*, it says Takefu SG2 is used by Shun and Fallkniven. Takamura and Itou use Kobelco Steel R2.
So, are R2 and SG2 the same steel? It depends. If we're talking about Kobelco R2, then, yes, they are the same. If we're talking about GSB Acero R2, then, no, they're not the same.
Also, if we're talking about Kobelco R2 and Takefu SG2, in deciding on which steel, then, what seems to be important is whether or not the maker forges and heat treats the knife, or uses stamped steel.
I hope that helps.
Although, I'm not sure anyone cares at this late date.