Hello, there should be caches present on the website for you to purge it, so command curl -X PURGE https://site.com wont work blindly on each and every subdomain. You have to first confirm if there are caches or not, to do so you have to do the following command

curl -s -D - https://site.com -o /dev/null

For complete reference I have also added a report in the writeup : https://hackerone.com/reports/154278

I hope this have solved your doubts😊