I'm going to dumb this down for you, friend.
First, it's not character "assassintation," as you call it, if it's truthful.
Next, Jonathan Greenberg is referred to as a "former writer" for Forbes because he doesn't write for Forbes any longer. "Former" means "previous." He was, though, a writer for Forbes when Trump, as John Barron, called him and lied his way onto the Forbes 400 list. The Post's story was from former (previous) Forbes writer Greenberg's first-hand account.
Trump has admitted to using aliases in past dealings with the media.
I can't figure out whether you're alarmingly stupid or just nuts. I think I'll go with both. And don't accuse me of character assassination, danc. As I indicated previously, it's not character assassination (or assassintation, if you prefer) if the statement's true.