El Salvador is just taking advantage of certain conditions in the loan agreement that were ambiguous or perhaps flexible. Somehow, the country is able to skirt the condition that prohibited its public sector to further purchase Bitcoin.
As I'm interested how exactly El Salvador does this, I googled around a bit (AIs weren't helpful in this case).
I found
this Spanish article which lists several ways the government can buy Bitcoin without using taxpayers' money, citing an
X thread by "Bitcoin Beach":
1) Transaction fees and "donations" which could go to the Bitcoin Office. I guess they refer to fees in Chivo (the article unfortunately isn't that clear here, and the X thread also doesn't help, instead confusing me more - it mentions help by the US government ...).
2) Mining. The government could support private mining facilities in the country, which as a compensation would return Bitcoins to the Bukele government. Unfortunately the article is also not detailed here, but I guess this "support" could be done for example with tax exemptions.
3) Adquisitions by the private sector. This would be a similar "loop", e.g. the government "supporting" companies (e.g. via tax exemptions) and these companies also would transfer the coins to the government's or the Bitcoin Office's wallets.