Yes, it has been common lately for actors to lie about being cast in big roles, especially for Marvel Studios actors, but it is uncertain if that is the case with Colter. Charlie Cox did give a similar answer as Colter about his potential return as Daredevil, but that happened in April. Keep in mind that Charles Villanueva of Murphy’s Multiverse, who originally reported Charlie Cox’s return, had said that the deal with Cox was made “months” before the character rights clause ended with Netflix
In an interview with ComicSoon.net, Mike Colter confirmed that Marvel Studios has not contacted him “whatsoever” about reprising his role as Luke Cage, but that he wouldn’t be opposed to returning if asked:
“I got to say, I rarely think about it in the sense of what could be or will be, I know fans are eager to see something with the character…I know I established him and it was an honor to do that, but I don’t know what the future holds, I’m just in my mode of doing Evil right now and a few films on hiatus. If something happens, I’d love to have a conversation with them, but for now I’m not holding my breath, I’m happy either way, it was a good run.”
It is entirely possible that, at the time of that interview, Charlie Cox really thought that he wasn’t coming back to the role either and had no contact with Marvel Studios, only for them to call him a few months ago. At this point, it’s becoming less and less likely for any other Defenders actor to return in a Marvel Studios production any time soon, especially in Spider-Man 3.