The belief of the IRS is entirely separate from the issue of theft and bringing it up in this context is just confusing the issue. It's like asking "If they believe he earned it, and the sun is rising, is it before noon?"

Not necessarily. Taking somebody else's property is only theft if you didn't have a legal right to do it. For example, if the IRS levied his property to satisfy the tax debt, that would not legally be theft because the IRS is legally entitled to impose levies for that purpose. It also would not mean that they didn't think he earned the levied property. It just means that they wanted the property and had a legal right to take it.