Tacitus was a late Imperial historian writing in a period of relative peace, and known to be subject to exaggeration and outright flattery at times to ingratiate himself with the powerful.

That being said, he was most definitely talking about garrison troops. Legions did occasionally travel with substantial swine herds to eat, but nowhere near the scale of 1 hog per day per 40 men. A figure of around 1 hog per day per 250 men might be more accurate for an army on a prolonged campaign.

Note that few legions in Tacitus' day actually went on such long campaigns. The legions were generally fairly sedentary at the time, only marching relatively short distances to deal with border incursions.

The Dacian Wars towards the end of Tacitus' life were one major exception to this rule: and although the legions were exceptionally well supplied during them as Dacia was quite close to major Greek and Illaryian holdings of Rome (and connected to them by navigable rivers), you'll likely find that meat consumption for the actively canpaigning legions even then fell substantially below this figure of 1 per 60 if you look into any reliable statistics available for the wars.