Article 9 does not prevent arms exports. It says absolutely nothing about them. The constitution is completely silent on the matter. Japan has no constitutional or legal bar on exporting arms. There is, in fact legislation on arms export permits.To actually make the Japanese Defence industry sustainable over the long term they are going to have to address Article 9 to allow exports of their local defence industry.
The export of weapons & dual-use equipment is prevented by a policy, not law or any part of the constitution. The policy is rather simple: the export permits required by law will not, in general, be issued. There are a few very narrow exceptions related to joint developments, all of which are currently with the USA.
In theory (it would be completely legal & constitutional), the cabinet could decide tomorrow to change the policy, & instruct the civil servants who are responsible for export permits accordingly, without reference to the Diet. In practice, things are different: such a move would cause a political storm.
I repeat this regularly, on this & other forums. Sometimes I provide a link to the Japanese Ministry of Foreign Affairs, which spells out the matter on its website, the official English translation of the constitution, also online, & the Ministry of Defence explanation of the sections of the constitution relating to weapons & armed forces.