To flesh it out, at the moment you look at the card in your draw deck, it's not an event. It's a possession. Since it's a possession, you can't play it via leofric. If it said, this card is considered an event, then you could, however, it just says you can play it when you can play a skirmish event, not as if it was a skirmish event.
When you play them you don't play a skirmish event, but that is a
feature of the text and not an issue. If they said "this card is considered an event" then wouldn't you expect it to be discarded from the void, as an event would? Play
Riders of the Mark, exhaust a minion, discard it -- that's playing the card as if it were an event.
You can play a condition at any time in the fellowship phase. That doesn't mean you can play one from your deck instead of a character when you play AWiNL.
If there were a Gandalf condition that said "You may play this condition any time you could play a companion," then why couldn't it be played? That's a great example because it takes the timing out of the question. Compare the wording of
Riders of the Mark to
Great Heart -- if the text of the Rohan cards were only telling us
when the card could be played, then
Great Heart would have to be written in a similar way ("You may only play this condition when you could play a skirmish event").
Gollum, Plotting Deceiver shows yet another way the Rohan cards could have been written ("During a skirmish, you may play this companion from hand"). That the Rohan cards deliberately mention events when they don't have to makes me think that it's not just about "when" but "what" as well.
An interesting question is whether you think these cards can be played during a skirmish at
Cavern Entrance*. They're not skirmish events and they're not special abilities, right? And yet obviously there's something skirmish-event-esque about them, distinct from just their ability to be played during a skirmish phase the way
Great Heart is.