Thank you for using the Support Forum. As it states in the article you linked
"This can be the result of the network blocking traffic across port 515 due to Macs connected wirelessly over the network.
Since the driver uses Line Printer Daemon (LPD) to connect to the printer, and LPD traffic uses port 515, in this situation the print driver will not be able to establish a connection to the printer.
To get around this, remove the printer and re-add it using the HP Jet Direct protocol. HP Jet Direct is the Mac equivalent of the Microsoft Windows RAW protocol. It uses port 9100 to establish a connection to the printer."
Basically it is because the HP Jet DIrect uses port 9100 which will usually not have the traffic that port 515 will. LDP along with other devices connect using port 515 causing too much activity on port 515 so the printing fails to transfer to the Fiery. Please consider contacting your support centre for further assistance.
According to xerox support website, I have read the following url.
I uses LPR port with Mac which is connected using wireless lan with no issue. However, on one day, all the Mac cannot print to the bustled Fiery, jobs are being held under the Mac Printer Driver and won't send to the bustled FIery. After I changed according to above information from the url (ie. change the port from LPR to HP JetDirect/IPP), then those Mac can then print to the bustled Fiery.
As such, I have concerns why this issue will happen? I would like to know the reason because I need extra efforts to change the ports on all the Mac from LPR port to HP JetDIrect/IPP on unknown reason. Can anybody help to find and explain the reason with this issue? Many thanks.