There has long been an exemption to the general arms export ban, for US-Japanese joint ventures. This is a matter of public record, not classified, though some of the specifics are not public. The Raytheon website, for example, openly states that the SM3 Block IIA is a US-Japanese co-development, & US DoD press releases state that this development builds on earlier joint projects. The delivery of Japanese AESA TRMs to the USA for testing in the 1990s has been public information (& officially, not rumoured) for at least a decade. But this is rather different from permitting Japanese manufacturers to export directly, & to a fairly long list of countries.
Note that co-production or co-development for Japanese use has never been restricted.