I think they already tried commercial off the shelf OCR software (which they didn't name but I would assume it's ABBYY) before they decided to build their own solution:
ABBYY hasn't been all that amazing in my experience. I compared it with Neat Scanner software a few months ago and the latter seemed to do a noticeably better job.
https://blogs.dropbox.com/tech/2017/04/creating-a-modern-ocr...