Glad to oblige.
The argument there would seem to hinge on the assumption that, on a legal tender-less market, money of a strictly unchanged supply would obviously displace all the others. If we assume as much, it would make sense to speak of inflation as ‘stealing’. But the issue with legal tender is precisely that we cannot know what kind of money would be preferred in a free market, so we cannot say for sure that inflation is stealing.
What if we assume that people prefer some money which’s supply increases at a steady and predictable rate? Would a central bank printing fiat currency at a steady and predictable rate be comiting theft? Of course, it would be guilty of imposing a legal tender, even if it somehow managed to produce exactly the same currency that would have emerged in a free market. But assuming such a miracle is performed, would they be stealing?
Now, I know that many (most?) here would find the concept of anything but gold emerging as the money of choice in a free market as ludicrous, but the fact is that this is far from an apodictically sure thing. It is an assumption, be it an easy one.
So, let those who consider inflation as stealing be very clear, that they think so because they do not doubt that the supply of whatever currency would emerge in a free market would be unchanged.
In short, inflation is theft only if a further assumption is made, while the imposition of legal tender is always immoral.