It depends on individual state laws. The National Association of Secretaries of State has produced this document (Oct '20) summarising the laws on electioneering activities within a certain distance of polling places in each state.
The case linked in the question occurred in Virginia, and appears to fall under §24.2-604(A) of the Code of Virginia:
During the times the polls are open and ballots are being counted, it
is unlawful for any person (i) to loiter or congregate within 40 feet
of any entrance of any polling place; (ii) within such distance to
give, tender, or exhibit any ballot, ticket, or other campaign
material to any person or to solicit or in any manner attempt to
influence any person in casting his vote; or (iii) to hinder or delay
a qualified voter in entering or leaving a polling place.
So given that the campaigners were 100 feet away, and didn't block access to the building, it would seem that they didn't fall foul of this statute, despite the fact that election officials quoted in the article "acknowledged that some voters and polling staff members felt intimidated by what some saw as protesters".
I'm not aware of any state statutes which would prevent these campaigners from being armed, and any such prohibitions would be likely to be challenged under the 2nd amendment.