The 1 year term describes the subsistence as indefinate, as it includes all the growing seasons \ harvests. If you only need to feed 1 person for a few months out of the year, the allocation would shift.
There is no "1 year term" in what he posted. He posted "per year".
A 1 year term would not describe something something over an "indefinate" amount of time, it would describe it over a definite amount of time - 1 year.
A "per year" term is indefinite - and irrelevant since the amount of land required doesn't increase or decrease over time (provided you have enough to rotate your crops, fallow, etc. as appropriate, and a single acre is plenty of space to manage that for a single person). The only way "per year" even means anything is if you're referring to the yield as in "how long can I live off of this". Farming up X years worth of food for stockpiling in a bomb shelter or some shit, for example. But you wouldn't do that for subsistence farming (eat as needed and sell/trade any excess) or any long-term farming (if X > 1 you better be selling your excess or you're just wasting it as it spoils).
"Per year" would only make sense if it were a one-time harvest of a specific crop or if you planned to not farm the land afterward / were preparing for a bad harvest in the future. In either case the "per year" term is redundant as you can simply reduce the acreage requirement by the same factor.
If you could get 3 years worth of stockpiled grain for a person out of an acre of land in a single harvest of an annual crop, you would just say the requirement per person was 1/3 acre per person, not 1 acre per person per 3 years, every year.
If you need to feed 1 person for a few months, then you would be talking about specific seasons and crops - you wouldn't mention "per year" at all.