I'm not asking for any enforcement here. Ideally, I'd like to see this section of the specification either removed altogether or rewritten as an "extra challenge" note. The phrase "Please use" is ambiguous—it's unclear whether the author intends to prohibit certain solutions or simply wants to guide us in a particular direction.

Both @Aneadiel and @dfhwze made valid, useful, accurate statements regarding the nature of this solution and how the testing suite has been designed to handle it.

to elaborate / paraphrase @Aneadiel: "they didn't solve the kata as expected because they used DDL"

to elaborate / paraphrase @dfhwze: "yeah, but there is no testing code that fails solutions that use DML, so any user that does so will get away with it"

THEY each respectfully contributed to the advancement of understanding of the situation from which to learn for all readers thereafter perusing this particular comment thread...

So, to bring you up to speed on to what enforcement refers, (or in this case the lack thereof); it's the code within a testing suite that specifically prevents solutions that do not observe the specs provided in the description from passing the kata.

It's a request, what enforcement do you want? Police to show up?

but it is not enforced

It specifically states to use DML not DDL

CPU goes BRRRR