Literally the next para in that blog - "we committed to retaining KPMG, the well-respected auditing firm, to audit our practices annually and publish a public report confirming we're doing what we said we would."

In addition Mozilla put them through a rigorous process when selecting them as their default DoH provider which included them contractually agreeing to adhere to their stated policy. https://wiki.mozilla.org/Security/DOH-resolver-policy