It says "no Party" - does it mean this only binds the signatories of the treaty - which probably are countries and not individual developers? And if so, I would read it as "we promise there will be no state law to mandate all software in the country be open-source" which seems to be reasonable (even though I never heard about any country trying to pass such a law anyway) not "open source licenses are un-enforceable". Also, it says "owned by person of another Party", so it's mostly even "we promise no import/export bans on non-open-source software", and the intra-governmental actions are not even discussed, only cross-governmental ones. Also (3) says if the government negotiates contracts, it still can require the software to be open-source, it just can't have blanket ban on non-open-source software imports altogether.

Am I misreading it or misunderstanding what "Party" means?