It doesn't matter what tools and facilities they used, they still designed and produced a web page. They are the ones who chose to use squarespace, and use that template, and be ok with the end result.

It's not like a pizza shop or a hairdresser who never claimed that their very reason for existing and mission and product was accessibility.

This is a completely topical and fair challenge.

Why did they need a WYSIWYG editor to lay out static content in an entirely standard format? That could be done in Markdown, or plain html. These have the advantage of being trivial to handle with accessibility layers.