isn't the point that anything that fork()s can't run?
With regard to forking it was specifically talking about tasks that fork in order to change their identity at runtime as is common in daemon processes that can act as or on behalf of many different users. Processes within docker can fork more generally otherwise how would a great many things (like bash at all) work?
The second point where FTP was specifically mentioned is the use of dynamic port allocation. That is a bad point as that is an existing problem for many firewall & related solutions, generally and with regard to running in virtual architectures, so not specific to docker at all. Depending on what you might want FTP for there are many FTP daemon options that don't need to do this, and better protocols supported by services that don't need to do this either.
Using FTP as a bad example here is silly. What compelling reason is there to want to run an FTPd for which there isn't a much better solution anyway? If you are wanting to run an FTPd then you either have a very specific requirement or you are stuck in the past.
FTP was a pain to work with back in the day when I wrote and sold client applications using it, when there wasn't an alternative for many use cases, but these days there are few (if any?) reasons to want to run an FTP server where there isn't a better alternative.