I'm not a business person, so take this with a grain of salt, but my understanding is that this is common for a lot of smaller startups and tech companies.

If you can raise funds outside of revenue (i.e. outside of directly selling your products), you can keep operating even if you're not actually generating any income directly. Typically that will be in the form of investment and loans. So even if your expenses (incl. repayments for outstanding loans etc) are higher than your revenue, you can stay in business as long as you can convince enough investors that it's still worth their while to give you their money.

I don't know whether this is true for Discord specifically, but I understand it's a fairly common strategy, especially for companies where their best chance of success is by being the only player in a given market.