No, in the described case, the message can get lost. This is a really unlikely scenario though. The only real-world case that I know of first-hand is using file buffer and somehow being unable to write to disk, possibly because the disk is full. Something like that can be prevented by a fairly routine set of server monitoring alerts.
It's in http://docs.fluentd.org/articles/high-availability#forwarder..., which says:
However, possible message loss scenarios do exist:
The process [log forwarder’s fluentd from the paragraph above] dies immediately after receiving the events, but before writing them into the buffer.
Is this document out of date?