This verse suggests that the Prophet (PBUH) does guide people,
Yet this verse makes it clear that he doesn't. Only God can:
So, did the former abrogate the latter, or vice versa? Nobody, to the best of my knowledge, has suggested that. They would be wrong if they did, since guidance is showing the road, which the Prophet, peace be upon him, could and did do. It's another thing to walk the road, which is what 28:56 speaks of.
And if you're saying that is an interpretation, you'd be right, but to use interpretation to reconcile apparent conflicts between verses and thus eliminate the need to claim abrogation, is IMHO a legitimate analysis and that reasoning has been asserted by many scholars as well.
But we don't even need interpretation of what guidance means in each verse. This is another example of human will operating under God's will. Just like what we will is free, but is always subject to God's will, so is guidance. One can do everything to guide somebody, but God does not let the person be guided. It's not random; it's based on God's knowledge of whom deserves guidance. This is precisely why God ends 28:56 with "And He is most Knowing of the guided."