I've had a look round there website and I can't see anything about an API. Can you point me in the direction of some documentation (or anything really)?
Specifically, 5e.tools includes full results (class features, items, spells, monsters, ...) from a large number of books, which are definitely not all open content.
the basic ruleset is free to use in commercial products under the OGL [1]. The official Wizards of the Coast published adventures are not free for distribution and is (legally) available only through authorized resellers like D&D Beyond and roll20.