I tested it in the middle of last year and it was very stubborn.
Today I made a second attempt. For a person who "something-knows" about Sigil plugins, talking to perplexity.ai was a pleasure. In a few steps we came to an agreement, AI was catching my suggestions on the fly, and within seconds I was receiving a fresh version of the plugin code.
I wasn't going to create a plugin to convert PDF to EPUB, but I asked for a plugin that would look up the email address in the EXIF description for JPG files.
AI did not insist on its solutions, it promptly recognized its mistakes, so – after several prompts – the plugin was ready.
I liked perplexity.ai (this is not an advertisement, I am simply stating the fact that I had a more pleasant conversation with this AI than with others).
I know that she is getting more and more "smart" with each new conversation.
So I have to admit that AI writing a working plug-in for Sigil IS possible