If you just played against this user and felt something was off, your instincts were 100% correct. This player has been caught blatantly cheating, and there is clear evidence—including their own admission—that they are using hacks. You're not the only one who noticed.
Here is what you need to do next:
1. Report the Account: Go to their Steam profile, click the ... button, and select "Report Player". Choose "Cheating in a Game" to ensure Steam Support investigates.
2. Block All Communication: Prevent them from messaging you or joining your lobbies by blocking them immediately.
3. Spread the Word: Copy the link to this Pastebin and share it on their profile or with other players in your lobby so everyone knows exactly what they are dealing with.
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To the owner of this account:
Your cheating has been documented. Your main account, along with any associated alt accounts operating on your network, have been formally reported to Steam for Terms of Service violations.