In fact, some of these arguments are waived on appeal if not made in a post-trial motion.
What you're saying is accurate but it's not the point. They are basing their new argument on "fresh evidence". To be successful, an appeal must be based on a legal argument and not on new evidence.
Source: http://www.americanbar.org/groups/public_education/resources...
The appeals courts do not usually consider new witnesses or new evidence. Appeals in either civil or criminal cases are usually based on arguments that there were errors in the trial’s procedure or errors in the judge's interpretation of the law.