FX necessarily trades in pairs. To buy a peso one needs to sell something against it, and the US will sell dollars if acting in the FX space.

Since the USD is one of the core currency crosses and is at the base layer of the financial system, raising the peso/dollar cross is effectively strengthening the currency against all other currencies.

Also, while the specific political circumstances around the pesos swap line are dubious, the US often extends dollar swap lines to other central banks. The other side of the privilege of having the world reserve currency is having the obligation to support the stability of the world financial system (although this example is more off the wall than the typical example).