The issue is that the requirement actually appears to be recognized by the game.
Before reaching 1852 SoloQ rating, the items were highlighted in red and unavailable for purchase. After reaching the required rating, their color changed to normal, which indicates that the rating requirement was fulfilled.
Additionally, the vendor requirement explicitly states that both 3v3 and 5v5 ratings are valid. SoloQ on Blackrock occupies the 5v5 bracket slot, so it should count toward the requirement.
If SoloQ rating was completely invalid for this purchase, the items should have remained red permanently. Instead, the vendor visually acknowledges the rating, but the purchase itself fails with a rank error, which is why this looks like a bug.
That explanation actually makes more sense, but in that case the vendor/client behavior is still extremely misleading, since the items visually become available after reaching SoloQ rating.
And even if that is how the system currently works, SoloQ is arguably harder than regular 3v3, because in normal 3v3 you choose your teammates and play with a coordinated team, while SoloQ matches you with random players every game.
Because of that, it would be completely reasonable for SoloQ rating to also qualify for purchasing this item.