No, of course he isn't.
I'd feel worse about running grossly-misleading traffic-bait headlines, but this IS basically Disney/Marvel's whole promotion-strategy at this juncture, so really I'm just playing the game...
I wonder... have "Firefly" fans accepted yet that Nathan Fillion is now "Castle" to about 98% of people who actually know who he is now? (Oh, but don't worry kids - that Trek-level syndication-juggernaut-followed-by-massive-pop-cultural-revival is still toooootally just around the corner. Totally.)
In any case, here's the "story": A month or so back, ABC Network president Paul Lee teased a "secret" about a "marvel superhero character" being somehow related to an episode of the series' upcoming third season, which most people figured was a jokey reference to the title character putting on a Halloween costume (he went "as" Mal Reynolds in Season 2, so there's some precedence for this.) Now, comicbookmovie.com - your #1 source for bizzarely-tantilizing non-news - thinks they've figured it out... and WHAT they think they've figured out is that it's not just a reference but a full-fledged 'real' character(s?)-cameo that would add "Castle" to "The Avengers"-continuity.
The "evidence": Lyle Lovett (seriously?) is listed on the IMDB as playing an "Agent Westfield" ("of S.H.I.E.L.D," presumably) in "Avengers," which is also the name of a vaugely S.H.I.E.L.D./Men-In-Black-esque agent character he played on an episode of "Castle" that involved a UFO/spy coverup.
FWIW, there's a certain amount of precedent for this: It's an ABC show, which puts it under the requisite Disney umbrella, and the series is big on "metafiction" - Fillion's title-character is supposed to be a mystery-novelist who teams-up with actual cops to solve murders (it's that kind of show) and ghostwritten 'real' versions of the books attributed to him are actually sold.