An inaccessible toolkit itself wouldn’t be illegal in the EU, but producing an end-user facing interface under the scope of the EU directive[1] (which as you mentioned is rambling but pretty wide ranging – “consumer terminal equipment with interactive computing capability, used for electronic communications services” could apply to most things that HN people work on) using that inaccessible toolkit would be.

But that’s just the EU. In the AODA[2] for example (which is already law), “barrier” is defined as “anything that prevents a person with a disability from fully participating in all aspects of society because of his or her disability, including a physical barrier, an architectural barrier, an information or communications barrier, an attitudinal barrier, a technological barrier, a policy or a practice” and sets out a process for the development / adoption of standards (the aforementioned WCAG 2.0 double-AA) and binds public and private sector to meet those standards to remove the barriers. Use of an inaccessible toolkit would certainly constitute “a technological barrier”.

[1] https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=uriserv%...

[2] https://www.ontario.ca/laws/statute/05a11