Hmm … I can’t repro with Obsidian 1.8.3 (on a MBP + MacOS Sequoia 15.2)
I’ve tested with the Native menus (under Obsidian’s Settings > Appearance > Advanced) enabled and disabled but what ever wikilink I right-click on, both options still appear…
Did you try to locate Open in Default App and Reveal in System Explorer in the Sandbox Vault ? (Restricted Mode toggled On + Default theme, etc…)