I don't think the term startup means that you have only a few employees. Rather, it means you are still researching your business model, still in a growth stage, etc. Certainly Tesla and SpaceX are startups, despite having thousands of employees. Tesla is experimenting with home batteries, licencing tech, SpaceX considering internet satellite constellations, etc. They are both very far away from achieving their goals, and stability as revenue-generating, competive companies. Definitely still startups.

Same goes with Uber, etc.

Startup = Growth - cf Paul Graham ;)