Quick glance of their page only mentions Turris OS being built on open source. If I can't blow away Turris OS and install whatever, then it's not open and uninteresting.

Vanilla OpenWRT runs Omnia (Marvell Armada). OpenWRT support for IPQ9574 is WIP. Omnia NG is IPQ9574.

https://openwrt.org/toh/turris/turris_omnia

Turris has its own OpenWRT warapper, but you can just wipe it and install the stock OpenWRT.