I am not from Microsoft or a partner.
This situation has happened before -- a product's media sometimes becomes available before its licenses.
The solution has usually been to buy the previous version's licenses with Software Assurance. (Jargon term: the L+SA).
So the solution for Exchange Server SE would be: Buy Exchange Server 2019 L+SA.
In some volume licensing programs (such as the Enterprise Agreement) this will happen automatically, because the program only sells L+SA.
What concerns me is the two-month delay in publication of product-specific license terms. The Exchange team does a superb job answering all kinds of questions on this forum, and they have listed what changes they planned for the licensing terms. But this forum is not a contract written by Microsoft attorneys and agreed to by customers. So there is a tiny, tiny risk that someone at Microsoft, but not on this forum, put something bad in the licensing terms that we will not see until we have been running Exchange Server SE for months.
There are solutions for this problem. But the best solution would be to have the final contract terms written down before the product must be put into production.