tl;dr - By dressing up as Tootsie, Hoffman suddenly realized how difficult it was to be a woman, to be judged - in this case, judged harshly - almost exclusively by one's appearance. He realized that he had been "brainwashed" to ignore unattractive women.
First of all, it's bizarre that Hoffman claims he didn't understand the struggle of the unattractive woman before. This is a man who is very frank about the fact that he didn't get roles because he wasn't handsome enough, or did get certain roles because no classically handsome man (his example is usually Robert Redford) would take them. Seriously, he talks about it ALL the time. But I'll give him the benefit of the doubt and assume he was just a self-centered, passively sexist man who just expected the women around him to be either stunningly beautiful or not worthwhile. That's fine.
But why are we so excited that he figured out that this was a shitty way to think? Why congratulate him for realizing that sexism exists, and that it hurts? Probably for the same reason that he got to spend all those pre-Tootsie years never thinking about it - his male privilege. When women point out pernicious, everyday sexism, they're either ignored or berated for being too bitter. When men do it, they're hailed as appropriately sensitive soldiers for gender equality. Apparently sexism doesn't exist until men say it does.
Also, I would just love it if famous people - men and women - would stop assuming they understand someone else's life after they dress up like them. You can't put on a lifetime of experience like a costume. Dustin Hoffman does not know what it's like to live as an ugly woman. Gwyneth Paltrow does not know what it's like to live as a fat person. Zoe Saldana does not understand Hollywood's racism against dark-skinned women because she played Nina Simone. They all get to take their costumes off and go back to being successful, powerful people.
By conflating a few weeks in drag with real life, they cheapen the real experiences and struggles they pretend to understand. And by congratulating these actors for their new "understanding," we diminish those real experiences and struggles right along with them.