Again, the Enclave Clause regards the District of Columbia. And the portion of Article IV, Section 3 that is making people struggle is, “...nothing in this Constitution shall be so construed as to Prejudice any Claims of the United States....”
No the 10 square miles refers to DC. There would be no need to talk about forts, arsenals, dockyards etc. in land already owned in DC. The enumerated purposes spelled out in the remainder of the enclave clause refers to land held by the inidiviual states which may be sold by state legislatures to the feds for those specifically defined uses.
Puts an end to the type nonsense we see in the West where certain states have become nothing more than vassals to DC as they’re in effect owned by the feds. AS I noted earlier, the feds can’t owne these vast swaths of land for non enumerated uses.
FEDgov is limited to very specific enumerated powers, everything else is vested in the people and states.