I think that person was mistaken. There might be Thalmor agents who the Legionaries have to help but they would not themselves be wearing elven armor. Why would the Thalmor gift them with that and waste its resources?
It did happen closer to Skyrim. I was probably A Mede who made that truce with the Thalmor but it certainly was not Titus Mede unless the guy is hundreds of years old. It happened around 30 years before Skyrims events so about 4E170ish. Watch the sounds of Skyrim. It says it plain as day on the script.
The person stated that they were Imperial Legionaries, and since they were wearing Elven Armour the other main way they would be identified as Legionaries would be through NPC name pop-up. If they are labelled as Legionaries I don't think they would actually be Thalmor agents. I don't think it's a waste of their resources either. Making your enemy wear your uniform instead of their own while still on service would be a huge insult and symbol of your dominance.
So if it happened closer to Skyrim, it didn't happen in The Infernal City, right?

Sort of confused about this now.
EDIT: I did watch the Sounds of Skyrim and read the script, but I don't know what date The Infernal City took place in so I can't tell.
EDIT 2: The elven armour thing was sourced from http://www.gamesas.com/index.php?/topic/1251475-anyone-encounter-or-see-the-thalmor-yet/page__p__19061724__hl__imperial+soldiers__fromsearch__1#entry19061724 thread. It states that they are named "Imperial Soldiers".