Very, very false, at least in any legal system based in English law (so the old colonials).  Nothing is illegal until explicitly made so by law, though that can include common law not necessarily a statute.  

A point this debate needs is that currency is not the same as legal tender.  Example, Scottich bank notes are widely recognised as currency in UK (with funny looks in England) but are not actually legal tender even in Scotland.  The exact definition of the term legal tender differs from country to country precisely because its the local specification of what is legally allowed to be used to settle a debt (not limited to tax).

I doubt Bitcoin is legal tender in any country unless there one has a open ended definition.  This does not mean you cant use Bitcoin, it just means its not recognised by law.  One receiving Bitcoins as payment for a trade could later say they have not been paid, they could technically say the Bitcoins have been held as security in lieu of full payment in legal tender.  

Another falsehood that the suggested FBI action to auction off their seized Bitcoins makes it legally recognised somehow.  This is no different from them selling off a car, house, boat, painting or anything else seized, they are simply selling an asset.