Strange the study might have used a placebo group of people infected with parasites. I guess they needed to test if their process actually was effective in generating hookworm infections?
They were testing a vaccine to prevent hookworm infection. [0] After giving the experimental vaccine, they intentionally inoculated the research subjects with hookworms to measure whether the vaccine prevented hookworm infection.
Yes. While today, the more common meaning of the word is "to introduce immunologically active material (such as an antibody or antigen) into especially in order to treat or prevent a disease", an older but still used meaning is "to introduce a microorganism into".
[0] https://twitter.com/JimmyBernot/status/1354443850153144321