Transcripts of two completely different countries’ conversations being released at the exact same time means the leak is US. To assume something else is silly.
Just because they were released at the same time doesn't necessarily mean the WaPo received them at the same time.
Let's say for example that the Mexican government furnished a copy to the Mexican Ambassador in Washington. Then a secretary or code-clerk at the Mexican Embassy could have mailed a copy to the WaP.
Something similar could have happened with the Australian government's transcript of the Prime Minister's talk with DJT, meaning among other things that the copies could have "come over the transom" at the WaPo at very different times.
But at this point, as HRC might say, What difference does it make? Either Pres. Trump said the things in the transcripts or he didn't. And if he did say them, then some of his supporters may not be entirely pleased.
The thoughts of who made the tape(intercept) are really moot. All that is needed is to get the person who published the transcripts to talk. this is Classified. There is no Press Priviledge or media shield Law. The publisher will squeel and all will be released.
Just like everything else, therein lies the problem. Team Trump doesn’t want to know. Just like he won’t ask Sessions to do his job.