Can take filesystem bkp of pg data folder when server is running

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/10/static/backup-file.html
Description:

I am running postgresql 10 on mac and was experimenting on backup and
restore features of PG 10
I certainly can take FS backup of the cluster with TAR when server is
running and with ACTIVE client connections [dummy begin; statement]

This seems against the documented explanation for FS backup method and its
limitation.

Is there something I am missing here?