This is incorrect.
Metrolink runs to San Clemente every day. Only San Clemente Pier Metrolink Station is weekends only. Both San Clemente Metrolink Stations are right on the beach.
You can still take Metrolink to the beach on a weekday, but it will be more expensive than the $10 Weekend Day Pass.
If you are looking into using public transit to go to the beach, you could also take LA Metro Bus 460 from
Disneyland to Knotts Berry Farm then transfer to OCTA Bus 29 to Huntington Beach. Once in Huntington Beach, you can explore that beach or take OCTA Bus 1 down Pacific Coast Highway to more beaches.
https://www.metro.net/riding/maps/
http://www.octa.net/Bus/Routes-and-Schedules/eBusbook/eBusbook-February-2016/