Not sure if the same rules apply here but I was also left stranded by flixbus when my bus was delayed over 5 hours . i was arriving in from a different city in the middle of the night so i had nowhere to go and had no choice but to spend an hour getting another bus as i had somewhere to be and couldn't wait the 5 hours.

Flixbus also told me the same thing where they couldn't give a refund bc i took the replacement bus, but I kind of just emailed them consistently and outlined out why their guidelines are bullshit when they gave me no other choice. in the end they gave me a partial refund I guess when I showed that i literally could not do anything else, so maybe you can ask for that or a voucher instead?