I recently interviewed for a company and after the interview I sent a thank you email to the interviewer. He replied saying they are going to interview a few more people but I did a good job and I could get started by reading some articles to familiarize myself with their stack. If he is asking me to learn more about the company does that mean I got the job? But if I did, why would he still be interviewing more people?
This is a common theme for employers. They want you to be up to speed as much as possible so you don't waste time
Employers are not in it for your feelings, they want the best possible person. The fact they said that means they're optimistic about you, but if they find a better candidate they will most likely go for them.
Before any interview you should look at the company, their website, understand their stack, etc. Having knowledge of who they are, what they do, why they do it, the principals they stand for, mission statements etc. are all critically important for some employers.
Employers also want you to not look for other work if they like you, so they will try to lead you on even if they have only a slight intention of having you employed.
My suggestion: Spend 5-10 minutes on their site and quickly jot some notes down on it and the stack they use, then keep looking at jobs. Don't stop looking until you get your job. And, before your next interview, spend time on the next companies site, and learning their stack. It gives you more ammunition for the interview, and let's you focus on specific points, especially if they have mission statements and values.
His comment might simply mean that they're hiring a bunch of people and you're one of them, however that's just guesswork. You don't have a job until you've signed a contract confirming it.
As for "familiarizing yourself with their stack" ... that's what you do once you've been hired. Why would you do a bunch of work without getting reimbursed for it? (presumably you already know what they use in their shop because you've interviewed for the job, I'm talking about actually getting started on tutorials for those technologies which you might not be too familiar with)
Keep looking, and don't count on anything until you have a contract in hand.
What it means is that you are not eliminated from consideration and he feels you are a strong candidate. He is interviewing more because he has interviews scheduled and some of them might be better candidates than you are. It is very rare for a company to find someone they like and then cancel all existing scheduled interviews.
Most cynically: He thinks that if you do some work for free before you start, it will be good for him, and if he doesn't hire you, it won't have mattered to him anyway.
Still pretty cynically: be ready for further technical questions, possibly off the cuff. Showing you are familiar with these technologies - more than the other candidates - may make a big difference.
It's your choice how much prep work you want to put into this interview process. If you really want the job and have time then work harder. Et cetera.