A Splash-page can also be used with an SSID that is configured for PSK.
You don't need a RADIUS-server, but there has to be an instance that delivers the splash-page (a web server) and a server that gives the feedback that the user interacted in the "correct" way with the splash-page. The later could be a RADIUS-server, but you could also use the Meraki cloud without a RADIUS-server.