But since the latter is "Covert", we'll never know if it's the case and we can't go straight to conclusion based from that information alone.
My best guess is Foundry USA doesn't rely on SPV mining since it's for "large miners" (more info about SPV Mining)
and/or they have an extremely good connection that their miners receive the newly mined block at a very low latency (almost instantly) so even the next lucky block they mined in seconds can contain transactions.
-edit-
Typo