The bullet point for serviceAccountName through the whole page [1] (there are 11 occurrences), mentions:

"The service account is only required in multi log forwarder implementations if the log forwarder is not deployed in the openshift-logging namespace."

* which is not correct * discussed here [2]

[1]https://docs.openshift.com/container-platform/4.15/logging/log_collection_forwarding/configuring-log-forwarding.html

[2] https://redhat-internal.slack.com/archives/CB3HXM2QK/p1709139340516329

Docs from 4.12 and forward.       

"The service account is required whenever the CLF is not named 'instance'"