To answer the question: The professor assumes the email referred to 1900-1910 with “late 1900s”. As this was normal 20 years ago (and still gets used). He then gets upset realising the age difference between him and his student was likely the main contributor to this incorrect assumption.
To ask a question back: From https://www.bucknell.edu/fac-staff/john-penniman, I read:
John Penniman is Associate Professor and chair of Religious Studies
I would say for religious studies it should be fine. But also for other areas, why can’t you use 1994 papers?
https://maps.app.goo.gl/hPQahGuQQpM8NUZv5 In case we wanted more information, here they were eating