From my experience, yes, the busses only run to replace the Epcot beam when it is out of service due to scheduled maintenance or an issue.
On my last Poly stay, the Epcot beam was not running until 9am due to a scheduled closure. Epcot opened at 9am. I was able to catch a direct bus and was waiting in line at the Epcot entrance by 8:30am. The digital wait time also reflected busses were running to Epcot this morning.
When I was at the GF in December, something closed all the monorails while I was on the resort line. We were evacuated at MK, took the boat to TTC, and were ushered by CMs to an area loading people onto two Epcot busses.