The advice was technically illegal and dangerous / unwise as it exceeds the legal domestic storage limit.
However it depends how you take the message, whether you take "jerry can"
1. As specifying a 5 gallon metal container,suitable for the storage of petrol
2. A generic description of a standard petrol can
It seems the official European definition favours No. 2 with no mention of volume.
"Jerrican" means a metal or plastics packaging of rectangular or polygonal cross-section with one or more orifices
http://www.unece.org/fileadmin/DAM/trans/danger/publi/adr/adr2011/English/Part1.pdf
