Shitas Munbaz (Shabbos 68b)

I think that Munbaz' shita is a little confusing in the gemara. When I first read the gemara on 68b, I understood it to mean that Munbaz says you're only chayav a korban chatas if there is shigegas korban but not if there is shigegas lav or kareis. Here is the shakla v'tarya courtesy of Kollel Iyun Hadaf:

1. The case is, a baby was captured by Nochrim or a convert converted among Nochrim; he is also liable one Korban for eating blood, and one for Chelev, and one for idolatry [and for every other Chiyuv Chatas he transgressed].
2. Munvaz exempts.
3. Munvaz: [Regarding Shevu'as ha'Edus, one who transgresses] b'Mezid is called Chotei, just like Shogeg - just like Mezid knew [that he sinned], also Shogeg [is liable only if he once] knew.
4. R. Akiva: If so, you should say that just like Mezid knew at the time he sinned, also Shogeg who knows at the time he sins [is liable]!
5. Munvaz: Answer: Indeed, I say this! (This will be explained.)

The gemara will later explain that this is referring to shigegas korban. This sounds like Munvaz is saying that you are only chayav if you knew about it when you sinned and the only thing you didn't know was that you'd be chayav a korban if you did it b'shogeig. However, Rashi on the spot does say, "tchayvuhu gam b'zu chatas." I missed that when I read it though b/c to me it seems pashut pshat of the gemara is that you should only be chayav in that scenario just like line 3 from above. However, the gemara later makes it obvious that Rashi is correct. The gemara quotes the braisa on the bottom of 69a and attributes it to Munvaz where it says that you're chayav a chatas for shigegas korban and for shigegas lav. It fits in with the gemara on 68b but it just wasn't obvious to me when I first read it.