Yep... SoL doesn't even pop up in landlocked city build menus. So, the three basic requirements are: at least one steel resource (from mill or ironworks), less than 4 wonders already built in your city, and coastal.
There are several wonders that have 'hidden' requirements to build... the mod is inconsistent in whether they are displayed in build menus as greyed out with a tooltip explaining why (lack of prereq building like a university), greyed out with no tooltip explaining why (often a prereq or resource missing), or just absent from build menu (SoL, some of the science wonders if you already have another science wonder in that city).