Student or Learner
According to Longman Dictionary (http://www.ldoceonline.com/dictionary/in_1):
after followed by a time period is more often used to talk about past events
• After a few minutes he stopped.
in followed by a time period is more often used to talk about future events
• He'll be here in a few minutes.
However, I've come across 'in' in a past structure in the book Jesus of the Future by Alexander C. Parker (page 188):
... JESUS leaned toward him and started to talk to him in a very low voice, so only that man could hear what was being said; in a few minutes HE stoppped in front of another leader and said ...
I feel confused. Could you please tell me which one I should use?
"Invading armies have no rights." Noam Chomsky
Your reference does say "more often" in both entries. Going with the majority is a safe bet.
I am not a teacher.
I find the author's use of "in" unnatural.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.
But more often does not mean that this is the only possible meaning. I agree that this is an odd sentence, though.