But Ukraine is not a member. There is no reassurance required, or given, by NATO supplying non-members. In fact one could easily make the opposite claim: NATO depleting its own ammunition stores is doing the opposite of reassuring its members, by decreasing its own margins of safety.

Assurance to the other Eastern European countries that are members (read my previous response). The other Eastern European countries want to supply weapons to Ukraine (they have gone above and beyond to send extra to Ukraine).