Which legal professor?
Eric Banks, lawyer and former St. Louis City councillor, has the opposite opinion:
St. Louis Couple Points Guns At Protesters — Was It Legal?
No, it actually isn't true.
I was operating with some bad assumptions before: I assumed that Portland Place had a right-of-way that was a distinct property that happened to be jointly owned; turns out that this isn't the case... which is actually important: the property extends out into the middle of the road.
This means that there was an easement - either de facto or on title - granting access across the roadway part of the McCloskey's property. This is how the McCloskey's landlocked neighours - and their servants, deliveries, garbage pickup, road maintenance crews, etc. - were able to access their properties without the McCloskeys threatening to kill them.
So even though the road in front of their house was private property, it seems they didn't have the right to try to deny access.