the Rambam forgot a gemara?

The Rambam writes in Shegagos 7:6:

עשה תולדה של אב זה ותולדה של אב זה בהעלם אחת. יראה לי שהוא חייב שתי חטאות

If you do two different tolados from two different avos, the Rambam says "yireh li" a chiddush din that you are chayav two chata'os.

It's pretty rare for the Rambam to write "yireh li."  I imagine you can find in one of R' Chaim Kanievsky's seforim exactly how many times he says it, but even without knowing the exact number, we all know it's not too common.  The Rambam usually sticks to bringing down only what is explicit in sources in Chazal.  What is striking about this halacha is not only that the Rambam says a chiddush of "yireh li," but that he didn't have to say it -- the halacha he paskens is mefursh in Bava Kamma daf 2.   You could be learning in the slowest moving shiur and would get this far : )

ומאי איכא בין אב לתולדה נפקא מינה דאילו עביד שתי אבות בהדי הדדי אי נמי שתי תולדות בהדי הדדי מחייב אכל חדא וחדא

The meforshim have all kinds of creative solutions as to what the Rambam added that is not in the gemara, or why the gemara does not mean what it seems to say or might not be aliba d'hilchisa.  I don't see how you can say any of those wonderful answers when the Rambam himself was asked by his son what he meant here and he replied.  What he says is remarkable.  Quoting Kesef Mishneh:

ומ"ש עשה תולדה של אב זה וכו' יראה לי שהוא חייב שתי חטאות. מצאתי כתוב שנשאל ה"ר אברהם בנו של רבינו למה אמר יראה לי והלא גמרא ערוכה בריש ב"ק מאי איכא בין אב לתולדה נ"מ דאי עביד שני אבות בהדי הדדי א"נ שתי תולדות בהדי הדדי מיחייב תרתי א"כ מאי יראה לי דקאמר. והשיב לא אמר בגמרא שתי תולדות של שני אבות ולפיכך אמר יראה לי שאפשר לומר דהאי דאמרינן בגמרא שתי תולדות של אב אחד לאו אליבא דהלכתא ואפשר שנתעלם זה המקום ממנו ז"ל בעת שכתב יראה לי ומ"מ הדין אמת ואליבא דהלכתא עכ"ל.

In short, the second answer is that that maybe the Rambam simply forgot!  I can't recall seeing something like this, but maybe those of you who have more bekiyus will point me to other places

What confuses me is that if I'm not mistaken, the Rambam did multiple drafts of Mishneh Torah.  You would think that at some point in looking back at what he wrote he would have remembered and fixed the mistake.  In any case, it's good that the Rambam himself said this, because if you or I had said it we probably would have gotten thrown out of the beis medrash.  Whenever Tosfos asks questions from all over sha"s on Rashi, instead of looking at a Pnei Yehoshua or other meforshim, life would be easier, albeit less intellectually challenging and stimulating, if we could say sevaros like "Rashi forgot."  But that's just not done, unless I guess a Rishon does it for us.

You could highlight a different angle here and emphasize the gadlus of the Rambam: even though he did not have the sugya in mind, he still was mechavein to the right halacha.  There is an intuitive sense of halacha that great talmidei chachamim have that goes beyond knowing sources.  I remembering once hearing that the Chasam Sofer would pasken shaylos between Mincha and Ma'ariv and only afterwards write out the teshuvos.  He trusted his sense of what the outcome would be even before he consciously mapped out how to get there.