Valamis development started in 2013 so many decisions predates these ADRs. Some of those are still valid, some deprecated. Most important foundational architecture decisions are listed as ADRs.
Liferay was chosen as a platform.
Current Liferay-based foundation architecture in a nutshell:
Liferay is a customizable Java portal platform that provides functionality like CMS, authentication, development APIs and many integration possibilities. It makes it flexible choice for intra/extranets and websites. Valamis can then leverage the rich feature set of Liferay.