I would suggest that the manager probably made the offer, told other people and they said "so how was his code sample?" When the manager replied "I didn't ask for one," this same person gave him a lecture about not hiring someone without seeing a code sample.
Could be his boss or it could as easily be people you are going to work with. Either way, he now has to provide one to somebody, and he's almost certainly hoping like hell that you don't either say no or provide something poor.
So the question for you now becomes: is it worth causing him problems? That's not a great way to start a working relationship and if you have a code sample that you have faith in then there isn't really a problem. If, on the other hand, you don't have anything handy then tell him that.
Remember though that in most countries, you don't have any rights even in the preset probation period. You certainly don't have any before you even have an offer. You're not going to be able to say that you've been given an offer and hold them to it.
So I guess another question you have to ask yourself is: Does this put me off the company? I can't help you with that but I can tell you that I've seen similar (though not quite so serious) clerical errors in perfectly good places to work. It is a sign of some disorganisation, but it's not the worst red flag.