The US has laws about designating foreign terrorist orgs, but nothing about domestic. "Domestic terrorist organisation" has about the same legal meaning as "group of super duper bad guys".
This decree contains no enforcement powers other than to "enforce existing laws". We are not yet at the stage of authoritarianism where new crimes can be created by executive order.
The same decree was also done during Trump I, but I'm not aware of anyone being prosecuted for "antifa" membership.
TLDR: this proclamation is weak.