Command

ffuf -w /opt/useful/seclists/Discovery/DNS/subdomains-top1million-5000.txt:FUZZ -u http://academy.htb:PORT/ -H 'Host: FUZZ.academy.htb'

We see that all words in the wordlist are returning 200 OK! This is expected, as we are simply changing the header while visiting http://academy.htb:PORT/. So, we know that we will always get 200 OK. However, if the VHost does exist and we send a correct one in the header, we should get a different response size, as in that case, we would be getting the page from that VHosts, which is likely to show a different page.