Its talking about bittorrent clients not protecting itself properly. Also the article got this wrong the vulnerability is NOT in tor itself but the client using it.
This quote is too vague
The user's BitTorrent client then attempts to make a data connection
directly to the honeypot without using Tor, thus revealing the user's
IP address to the honeypot.
But the DHT part is something done over UDP which isn't protected yet the client still tries to make a connection to it. I skimmed the paper the article linked. It mentions clients connect to peers directly which sounds like nonsense or clients completely ignoring proxy settings. It then talks about analyzing traffic through tor so i believe clients do get some of it right.
A malice exit node isn't the same as turning off a firewall. Its more like visiting a site thats trying to use an exploit in your browser or phish you. The exit node adds extra peers to your connection and those peers are trying to figure out who you are. I didn't read it all so i cant break it down but thats the jist of it. Adding extra peers, hoping your client isn't implemented properly and it appears clients may leak information about other users/peers. The paper also mentions its not as easy when clients encrypt their data.