Well Lily and Marshall may be back together, it doesn't mean they will get engaged again, at least not right away. Remembering how they got engaged in the series opener - that situation put time pressure on the relationship, because they presumably had to get married at some point, and I was wondering if the show was actually going to go that way. If they are an unengaged couple, that gives more plot flexibility.
Funny but I wasn't expecting to see Tom Lenk in the episode. I was pleasantly startled when I recognized him!