The fate of Symbolics wound up having little to do with Stallman, Greenblatt, or LMI. Sun et al. just blew away Symbolics with the economies of scale of UNIX workstations, and the advantages of LISP didn't persuade many people to go with LISP machines instead.
Similarly, Rational Software started out making ADA machines, and only survived and prospered after a total pivot away from hardware and ADA alike.
Stallman has the normal sort of blindness most zealots have for areas outside of their zealotry. I have always found it a little bit fascinating to see what a naive view of the world he really has for anything that isn't the politics of software.
Which I don't say to be unfair, although I could see those who idolize him taking it that way.
I agree with your assessment but the comment you're replying to is making an untrue assumption, he didn't say that he sunk Symbolics himself or anything close to that. He said that he hindered their plan to drive LMI out of business for some time.
One of the huge problems of the Lisp maschines was that they where quite happy with taking money from military and other huge copperation and did not really try to compete with in that market.
They where also not quick enougth to get at least a subset of there software running on other hardware.
Similarly, Rational Software started out making ADA machines, and only survived and prospered after a total pivot away from hardware and ADA alike.