It does seem an irrational consideration - why is 30k BTC (less than $20m) labeled "such a large amount" instead of "irrelevantly small amount" ?

Even for BTC scale, this amount shouldn't be a big deal by itself. $20m or 30k BTC simply is not big in the financial market sense - it's a big amount for an individual or a single small company; but that's not a significant amount at all for supply/demand of any currency with supposedly millions of BTC in circulation.

If reality matches the published BTC market data, then 30k BTC should be a drop in a bucket with no meaningful change to the market. And if that kind of money actually does mean "a dump of a large amount", then the currently popularized BTC market estimates, capitalization, etc are exaggerating reality thousandfold.

Market capitalization is not the same as volume.

For example, bitstamp traded 16 million dollars in the last 24hs. The sale would mean as much as an entire daily trade! http://markets.blockchain.info/

To put it in perspective, Apple stock trades 71 million avg daily volume. At 90 dollars, its 6 billion dollars a day.

If someone were to sell 6 billion dollars of apple stock in one day, remain assured that the price would tank quickly as well, regardless of apple sales or cashflow or any of its "intrinsic" value. http://www.dailyfinance.com/quote/nasdaq/apple/aapl

One of small the altcoins - ReddCoin recently went from a market cap of ca. $600k to ca. $1m in two days, and everyone were inventing reasons for why. Except it was very simple: Someone had bought about $5k worth to pay his staff 1 week of salary in RDD (there was some tax advantage where he is to treating it as a "bonus" in something not recognised as a currency locally so they had an immediate advantage, and his company is crypto coin related).

Even for such a small coin, $5k was well below the typical daily trade volume. But here's the thing: He very rapidly bought off pretty much all the sell orders at 4 satoshi, and everyone started speculating what happened and prices shot through the roof, even though more coin than that enters the supply in no-time through mining still.

Point being that coins worth a typical daily trade volume can be enough to trigger large reactions in markets that are priced so extensively based on speculation about how other speculators will behave (take a look at the chat at various exchanges, and see the amount of chatter about which coin to buy into because they expect a pump or other forms of manipulation - very little of the chat seems to be related to non-speculative uses of the currencies).