The stock market is incredibly liquid: for any stock listed on an exchange, you can buy almost any quantity and sell almost any quantity, at any time the market is in session.
I think that sentence would make most professional equities traders chuckle. I mean, it's true-ish for some of the big etfs and some of the highest volume individual companies, but there are 8k+ listed symbols in the US and you can't just go buying and selling "almost any quantity" of most of them. Nobody really trades that way. Trading in size typically involves chopping a large order into smaller orders that get managed by a trader or really a trader's execution algorithm.
I'm not sure what exactly you mean by block (in terms of size), but let's say you have a stock and the average number of shares at the inside (best) price is X. If you are trying to trade 10X shares, you aren't just going to aggressively trade through multiple price levels all at once to get the trade done, and if you just place an order for 10X shares into the book you will find that the price starts moving away from your order price pretty quickly. For a high volume ETF like SPY, X can be thousands of shares, but for low volume mid/small caps it may be only a few hundred shares, which is not something I'd consider "incredibly liquid."
The sentence is close to literally true, but what I think you're questioning is the implied "[at a price close to the current price]" at the end of it.
I think that sentence would make most professional equities traders chuckle. I mean, it's true-ish for some of the big etfs and some of the highest volume individual companies, but there are 8k+ listed symbols in the US and you can't just go buying and selling "almost any quantity" of most of them. Nobody really trades that way. Trading in size typically involves chopping a large order into smaller orders that get managed by a trader or really a trader's execution algorithm.