I buy in to the idea that years are 'short', mostly because the years follow a lunar calendar, not a solar one. The world shows minimal effects of 'seasons', so it makes sense that a solar year wouldn't have meaning to them, but that leaves us no idea how long their year is relative to what we think of as a year. It could be remarkably shorter or longer than their rotational period. I think the dev note you're mentioning is the statement that years are made up of twelve lunar months, an Astral and Umbral month of each element. Theorized elsewhere in the Lore forums, though I don't remember where, is the idea that an "Astral" month centers around a full moon and an "Umbral" around a New Moon, which make sense. If that's the case, Hydaelyn's year is made up of six full lunar cycles, broken into Astral and Umbral months.
