I came across this statement in a Skuid tutorial about the Action Framework. It kind of confirms my suspicion that Guest force users may not have permission to use apex. You may want to check Salesforce documentation on that limitation.
Run Custom Apex Action
- Apex Action: All properly-annotated invocable methods will appear within the Apex Actionfield picklist; the first method that Skuid detects will likely be selected automatically. If you do not see your method, try refreshing the App Composer.
After selecting a method, each of its invocable variables will also appear as additional fields. As with other Skuid fields, invocable variables can point to various fields with the use of merge syntax.
Only authenticated Salesforce users may access invocable methods and run Apex actions.
Using this action type within a Skuid page on a public-facing Force.com site or community is not recommended, as guest users will not have the permissions for proper functionality.