Rav Hirsch explains Bo 12:2
הַחֹ֧דֶשׁ הַזֶּ֛ה לָכֶ֖ם רֹ֣אשׁ חֳדָשִׁ֑ים רִאשׁ֥וֹן הוּא֙ לָכֶ֔ם
That this was a method of giving the command, not that Moshe Rabbeinu did not understand the cycle of the moon.
Just as Hashem told Noah to turn his looks away from the earth which
had just been regiven to Man, and direct them to the heavens, and,
showing him the rainbow, said זאת אות הברית, this is tobe the sign,
the guarantee, of My covenant with the new future which I promise to
humanity, in the same way, in Egypt, at the threshold of the new
Jewish future, He called Moses and Aaron into the open, showed them
the silver crescent of the new moon, and said: "This renewal is to be
to you a beginning of New-moons" (literally a beginning of renewals,
of revivals), "and it, the month that is beginning with it, is to be
for you the first of the months of the year!". There are two מצות
contained in this verse, (a) קידוש החודש על פי ראית הלבנה בחידושה,
fixing the beginning of the month with the new moon becoming visible,
and (b) to make the order of the monts of the year start with Nissan,
the month of the redmption.
Indeed, the knowledge of the full cycle was already known and the calculation was used as the basis of determining when the witnesses were to be expected (as explained in Rosh Hashana, 23, 24, 25). That is why (for example) we see in the haftarah that we read for מחר חודש that Dovid and Yonasan knew that it was going to be Rosh Chodesh and that two days of Rosh Chodesh were kept at that time.
Thus we see that Hashem shows Moshe the first crescent of the new moon and tells him that this well known astronomical event is to be declared by witnesses and consecrated by the bais din. Otherwise, Hashem could have instructed Moshe to start from the actual conjuction of Nisan and alternate 29 and 30 day months or to use the calculated molad (as we do now) rather than waiting until witnesses can see the moon. . As Rav Hirsch says.
This is striking evidence that it is not the actual condition up
above, but the consecrating declaration of the representatives of
Israel that is the decisive factor on which the beginning of the month
I think the OP is asking about where the Mekhilta says (on 12:2) "החדש
הזה לכם וגו'... זה אחד מג' דברים שנתקשה משה והראהו המקום את כלן
באצבע", and the OP understands it to mean that Moshe didn't know till
then what a 'new moon' was. – @TamirEvan
If this is the case then I would read it as meaning that Moshe Rabbeinu knew exactly what a new moon was (the actual conjunction - when no moon is visible at all). Hashem was taking the opportunity to show him that the month would be determined by Bais Din on the basis of witnesses. I would also read it more like Rav Hirsch that Hashem was showing the deeper meaning of the new moon to Moshe Rabbeinu.