Fire Extinguisher 6 year maintenance
This seems to be a well documented maintenance procedure. I don't know much about fire extinguishers, but a quick internet search presented the following information:

Six-Year Maintenance
Every six years, stored pressure fire extinguishers that require a
12-year hydrostatic test (e.g. dry chemical extinguishers) must be
emptied and proper maintenance procedures performed [see NFPA 10(98),
Sec. 4-4.3]. The exception to this rule is nonrechargeable
extinguishers, which are required to be removed from service 12 years
from the date of manufacture. Again, this maintenance must be
performed by an approved extinguisher servicing company.

So it seems if the extinguishers are of the type that require a 12 year hydrostatic test, this appears to be a legitimate maintenance requirement. I guess it would also depend on if your locale requires you to follow the above referenced NFPA standard document or not.