You're thinking about to simplistically. The point wasn't always to alter the message, but to embolden and radicalize the target groups.
A reasonable communist might appeal to the people, you discredit them by making them more extreme. When the FBI and NYPD infiltrated the antiwar movement in the 60s, in many cases the agents were agent provocateurs, ultra-extreme "members" of these groups encouraging violence or other behaviors not in the interest of the group. The idea was to undermine and anti-war movement and identify the leaders and undermine them.
(Example: https://en.wikipedia.org/wiki/Gainesville_Eight "Bill Lemmer, the Southern regional assistant coordinator of Vietnam Veterans Against the War, revealed himself as an undercover FBI operative in May 1972. Bill Lemmer had been thrown out of a 'fast'/protest by DCVC(VVAW) on the Capitol steps in Washington D.C. in January 1972 after advocating for violent, destructive actions on the Washington Monument." )
If there is a covert presence on internet fora, it would probably be in a similar light -- build trust among potentially influential adversaries by emboldening them, then cutting them down by hook or crook. The ultimate coup is to actually co-opt the radical agenda.