It wouldn't surprise me, as faxes have some really weird legal exceptions carved out for them. For example, a faxed document is considered equivalent to the original, but a scanned image is not. So even though a scanned image signed with your private key would be much more verifiable than a faxed copy that is transmitted without encryption and printed in a shared office, the law considers the fax as better than the scanned image.
Which is all to say that it wouldn't surprise me if there were a legal requirement for an "original document", and if that requirement could only be met by physical mail or fax.
At least in the medical world the security requirements of documents at rest is much higher than ones in transit. So you can't easily do efax because as soon as you have a queued pdf its at rest. (It can be done. just not easily.) So when everybody switches over to voip they usually try to keep the old fax machines going which faxing over voip is hit or miss.
Faxes are (or were originally) point-to-point electronic transmissions -- one fax machine dialing via POTS to another fax machine. In addition, the received fax, and the response sheet received by the sender, have time and date stamps that give legally-admissible proof of transmission and reception (and the sender will get a report of failure if that's the case as well).
As a former fax server administrator (and troubleshooter/maintainer of everyday standalone fax machines), I think the point-to-point nature of traditional, non-Internet fax is the killer feature: no packets split up and resting on intermediate servers, etc. Fax servers have one or more individual fax modem cards, so the only network involved is before or after the actual fax transmission.
I know fax is outdated. There are other effective technologies to replace it. But I've found myself more than once speaking up as a defender of the technology because, as originally used and codified into law, it serves a specific purpose and, as a bonus, if you know how to use a photocopier and a telephone it's drop-dead simple to use.
No, patent applications don't need to be filed via fax. I've heard bad things about the various USPTO websites, so fax might have been the guy's preference as I can see it being relatively simpler.
(Again, like my other comments here, this is just my opinion, not that of the USPTO or US government.)
AFAIK email to fax (and vice-versa) converters have existed for a while ?
I'm much more annoyed that in 2022 we are still misusing pdf, treating it as a digital-first format rather than one more appropriate for archival of paper documents... (and with the associated "pdfs cannot be modified" myth)
PDFs are sort of lazy/arcane. Easy for author hard for reader. Layout your information for a print size and forever it shall stay. Bad for reading on most devices.
For those who are too young, a fax machine is this arcane device that used to be everywhere, like a remote photocopier.
Got the US patent dated 2013. Was the lawyer pulling my leg?