It’s the other way around. There is a six-year statute of limitations to civilly assess taxes under the 25% omission rule, but no statute of limitations for criminal evasion, which certainly appears to be present here.
That’s the way it was when I worked for the IRS in the 70’s and 80’s. I’m pretty sure the law hasn’t changed since then.
There is no Statute of Limitations on Civil Tax Fraud, only criminal tax fraud.
Here is a link to a great article explaining this in detail. I’m a retired CPA and this is the best simple explanation I have read on the subject.
“What is the Tax Fraud Statute of Limitations”