>what's to prevent the Azeris from just reneging like Ukraine did?
Presumably the Azeris actually want to follow through even if they hate the Armenis. Trade stops family fueds and age-long conflicts (see: 100 years war, Abraham Accords, Moroccan-American Treaty of Friendship, etc.), and this deal seems to be focused on promoting trade between Azerbaijan and its surroundings. Up until now Azerbaijan's only real ally was Turkey and Israel
since everyone else was part of Central Asian security organizations that hated Azerbaijan or trade organizations that hated Azerbaijan, and Armenia was making newfound friends of Turkey via Russia. They open back up trade relations with Armenia through Nagorno-Karabakh as a medium, and in exchange they presumably get to sit at the big kids' table and form trade agreements with countries that had otherwise cut off all ties with them in a few years' time and use Armeni infrastructure to transport goods to other countries.
Plus the Armenis just found a shit ton of Israeli spyware on their devices that was used against them during the last flare-up of fighting so they're getting to the point where they would rather work with the Muslims. Apparently even the Georgians are spooked by Armenia and Azerbaijan getting along and have agreed to allow Russian flights into and through the country severely angering the USA in the process
in order to rapidly start normalizing ties to Moscow out of fear of being the odd one out. Last but not least, Azerbaijan only won the last conflict because of drones, which Armenia is slowly buying from Iran and learning how to counteract TB2s from Russia, so the same trick probably won't work twice.