When people talk about threat to democracy or institutions, what they mean is threat to democrats or democratically run institutions. 

When democrats storm the White House or try to jail their political opponent or stage a coup on their own candidate, this is good for democracy because it serves the democrat party.

Let me try an oversimplification:

Storm the White House — Laws were broken and this was bad. (Although it would be more accurate to say “rioted near the White House.”)

Jail their political opponent — Trump broke laws, flagrantly, and prosecuting him is good.

Stage a coup… — Within the party’s rules. It’s a political question.