They're not claiming that mixing is required to let people withdraw securely or anything; it was never implemented before, the original SR never had it implemented the way that people assumed they did, etc. They seem to be claiming that attempts to implement the mixing led to technical problems which has buggered up withdrawals rather fundamentally.

With weeks having passed, it should have been easy to implement some sort of emergency measure that would let them transfer money back to people who wanted it. Instead they continued doing business as usual - money in, but not out - while implementing things like this withdrawal countdown system, which arguably seems calculated to try and maintain some confidence and keep the BTC rolling in.

It seems very likely that this is in fact a big scam, particularly given the shady behaviour of prominent and admin-affiliated vendors also happening, but the blockchain info linked isn't in and of itself definitive proof of anything much right now.