I'm no expert but it seems to me like it's basically the kind of thing that is only technically legal only because nobody has been stupid enough to push their luck.
That has been the Trump administration (and post-presidency) playbook since 2016, and it has worked out remarkably well for them (and shown how flimsy many of our laws are). I say send it and let the chips fall where they may. If the courts end up deciding "yeah, that's illegal" it's not like they can get the shells back, unless they want to remove them piece by piece from exploded Russian equipment and Russian soldiers. They just won't be able to use that tactic again. It's not blatantly illegal now.