Your information is correct, no car carrying ferries between Honduras and Belize.
The short road route described is by way of El Corinto Honduras.
From Pto.Cortes , Honduras follow the excellent highway west along the Gulf of Omoa.
Corinto has limited daylight hours of service for doing HN exit paperwork
On Guatemalan side there is only a small roadside Aduana kiosk about 10km
into the country and they will look at your papers and direct you to go to the full
service customs and immigration terminal in the Harbour area
of Puerto Barrios . Regardless of what they tell you , RIDE YOUR BIKE to that terminal. Some misguided official at the kiosk may say you have to take a taxi, NOT so!.
By the way , if you are on a trip to see C A why would you want to avoid riding through
such a nice part of it ?