In my opinion they are using their own platform extensively. Forum, marketplace, project management, cloud etc all is Mendix.
Regarding the website: Mendix is web-based, but not a website. In regards to SEO and content updates etc its better to use a tool as WP to publish webpages instead of an app.
Having said that: our website is a Mendix app though ;-) https://home.lowcodeacademy.com/link/home