That's one definition of liquidity (which applies here). In the context of companies liquidity can also mean: assets to pay short-term liabilities (based on the article, it seems Slack has no problems in that area).
> Stupid question but how do they create liquidity?
They just open for trading and hope that the buyers and sellers manage to find a reasonable price. (It works better if the company already has a relatively active "secondary market" - many of these private companies do, particularly if they grant stock to employees).
> Don't all the money go into the shareholders' pockets who sell in the listing?
Listing just makes it easier for shareholders to trade, yes. At some future point the company itself might sell stock (what would usually be called a "secondary offering" though technically the first one would be their IPO) or buy stock (a "buyback").
