1. High frequency trading firms with sub-millisecond latency, who make their money from the bid-offer spread. You can't compete here.
2. Medium term traders who will hold a position for somewhere between hours and weeks. They tend to be looking for statistical regularities to exploit. If they find one, they don't expect it to persist for long - the typical 'half life' of a strategy is around six months. It's not impossible for an amateur to compete here, but be aware that there are thousands (tens of thousands?) of people for whom this is a full time job. Many (most?) of them have backgrounds in quantitative finance, have worked at funds and large investment banks, have PhDs in physics, mathematics, computer science etc. So you should ask yourself why you think you will be able to compete.
3. Long-term traders who are either stock picking, or actively managing a portfolio that might include stocks, bonds, commodities, currencies etc. Some of them are just trying to beat the market, and some are looking for absolute return. All the points in the OP article apply to this case.