people don't "invest in" stable coins, but stable coins are used as a fiat equivalent on an exchange: people buy and sell into stable coins (or, more accurately, buy and sell into USDT because Tether is >90% of stable coin volume). As such, if Tether (the organisation) want to pump up the price, they can print lots of tether and use it to buy assets.

Something can be a security and not an investment. I would consider tether a security, but not an investment.

That's not what the Howie test says. In US law a security requires the expectation of earning a profit.

'Security' is an English word that exists outside US law, and refers to contracts in a more general sense than strictly 'investment'. I don't think OP was using the word that way, hence my response.

The comment I was responding too seemed to add meaning that I didn't read in the OPs comment. I may be wrong, but it was my intent to clarify based on my own reading.