He's just not desperate to sell and trying to catch some less informed buyer with too much btc.
These are rare after all.
The 2011 error coin is by no means rare, it is one of the largest mintage numbers for Cas coins, yet it sells consistently for a higher premium per coin than the 25BTC, which is indeed a lower mintage. There's just not as much love for the 25BTC than there is for the errors, or for most of the rest of the series for that matter.
The market for a 25BTC Cas is roughly 26-28BTC at the moment. Any user frequenting these forums is probably aware of that.
That user is just looking for a sucker who knows nothing about them and has money burning a hole in their pockets.