The definition of 'public' is not equivalent for IRC and for Slack.
IRC anope-style email verification simply requires that you are able to initiate a TCP connection to the IRC server. If you can do so, you can have a verified account. This kind of "public" is vulnerable to harassment as you describe.
Slack additionally requires an invitation by a workspace owner for all new users who wish to join, excepting only domains where the owner permits users to join without invitation using a special invite link. Anyone who invites harassers will find themselves under Slack's microscope as well. Owners who enable automatic invite approvals for e.g. @gmail.com or @throwaway.com that result in harassment can expect to find themselves banned by Slack and their Slack shutdown. While in theory this kind of "public" could be abused to some degree, it does not permit the 'anonymous' abuse that IRC's kind of "public" does.
If an IRC network was set up to require that existing users put their reputation on the line for new users, and in cases of harassment both the new and existing user were banned, then that would be a far more tenable system than what we have today. There's nothing that requires this to be unique to Slack. It just happens that Slack is willing to turn users away if they don't like it, and so as a result their network is baseline safer than virtually all IRC networks — such as Freenode.