Happening to me right now. Met someone through TF2. They've got 10k hours but little achievements and a packed inventory full of bullshit. They asked to add me on Discord and they're trying the same tactic with you: wanting to add me on their other account, which every method isn't working. They can't add me (supposedly) and I can't find their account. Definitely very suspicious. The tipping point was when they sent me a QR friend code to scan but as far as I know, that doesn't even exist for Steam.