It's a pretty common, well-accepted use in the hacker lexicon. See esr's Jargon File [0] where, by some sources [1][2], it started being used in its capacity as meaning 'understanding' for forty-ish years now at this point.

[0] http://www.catb.org/jargon/html/G/grok.html

[1] https://books.google.com/books?id=uS4EAAAAMBAJ&pg=PA32#v=one...

[2] https://en.wikipedia.org/wiki/Grok#In_computer_programmer_cu...