Then why is "longa" not in the accusative. The link demonstrates this, but doesn't explain why.
Like other sentences that contain "estas", there is no accusative direct object "-n". "Metron" takes the -n simply because -n is used with measurement. "-N" is used for a few other purposes that are not related to Direct Objects. Measurement is one of those other purposes.
According to Bertilo Wennergren (member of Academy of Esperanto): "The typical role of the accusative is to mark the direct object, but accusative has other uses too. That's standard terminology. "Accusative" is the name of a case. Cases have various uses.
"Somehow the idea that only N for direct object can be called "accusative" has spread a lot. It's however not correct." https://www.facebook.com/groups/duolingo.esperanto.learners/permalink/564207133741308/?comment_id=564211870407501&comment_tracking=%7B%22tn%22%3A%22R2%22%7D
I forgot to add that, personally, I have never seen a logical explanation as to exactly why a unit of measure would need to marked with an "-n". I do understand the logic of marking direct objects, and surely there must be some logic explaining the marking of measurements.
Perhaps it stems from Latin usage; see https://la.wikipedia.org/wiki/Accusativus#Accusativus_mensurae
However, it needs further evidence to state whether this usage of accusative in Esperanto was herited from Latin.
It would make sense... so why not! (Except, "longo" means length - "longeco" roughly meaning "the quality of being long" so is for remarking rather than measuring.)
I put in "That thread is one meter" snd Duo is like "Nu. You forgot the long."
Well, I'm glad I'm not the only one fretting about this. I only knew of two uses for the -n
1. direct object and 2. to show movement towards. I have also come across "Mi venos lundon" - "I shall come (on) Monday"