I noticed a website said check in will end at 10pm, is this for real?
I wanted to arrange an airport transfer to the resort but I don't see an email address or number
Yes, but let me help you out with a phone number +506 2479 7000
The check-in staff do leave at ten, and after that there's really just one overnight staff member to make sure the people coming in late at night have wristbands. If you're arriving late into San Jose (or elsewhere) I'd recommend overnighting there first and leaving the following morning. It's a fairly long drive anyway.
Get an answer from Arenal Backpackers Resort staff and past guests.