With regard to "tweets," as anyone can publish them, it's hard to get a good take on the content. It's nice that Robert Frost's poem, "A Secret Sits," is alluded to, short though it may be.

Stallone's line, "life is playing a in the dark with NO referee pal...", probably reflects nothing more than a typo. I have not seen slang used with "a." I have read some of Mr. Stallone's messages to get a sense of the kind of writer he is, and based on his style, I'm guessing it was an inadvertent slip.

Perhaps I assume too much, but if he "tweeting on the run," the way some of us text-message, that "a" may simply be slip of the...thumb, (rather than tongue).

Your question makes perfect sense, but I believe "a" is simply a mistake—though I wouldn't presume to tell you that I know for certain. Don't worry that it's "only" a tweet: the only "silly" question is one never asked. Above all, I hope this clarifies the text's content for you.