But where is the line? Can I import hundreds of people to manually have them dig coal out of the ground with their bare hands because I can't afford the machinery yet and claim I'm bootstrapping a mining business. We would typically call this exploitation.

Many types of business can't be bootstrapped. That is not unusual. Bootstrapping is primarily a popular approach now due to easy computer automation that makes a small subset of companies viable with the very small staff and no expensive hardware. It certainly isn't a default across all industries.

The labour pool is a market. You are not entitled to cheap labour just because you want it. If you can't find someone willing to do your work at the rate you want to pay it's because someone else is paying more for the same work, so you need to pay more to attract the labour.

Importing people from abroad is something that only exists as a solution because of the imbalance of development levels around the world right now. I expect that this is an anomaly that we are going to see correct itself in time once all countries reach a similer level of development. Where will you import your cheap labour from then?

In the UK we used to employ children to work in the mines and the mills. This was eventually realised to be exploitative and made illegal. I'd bet that all the business owners at the time complained how would they ever be profitable without the cheap child labour. But they either adjust or die, and I'm happy with that tradeoff. If a business is only viable with child labour I would prefer it didn't exist at all.