They’re taking the conventional approach – using the supremacy clause to argue that federal controlled substance laws and racketeering laws supersede the will of voters in Denver.
The problem with this line of reasoning is that we have historical precedent. It was called the 18th amendment (aka Prohibition), and it was repealed by state ratification in the form of the 21st amendment. In this case, cannabis isn’t part of a constitutional disagreement, but a specific law, so the anti-cannabis side has an even weaker argument.
The other issue with this line of argument is a majority of states (32) have legal medicinal cannabis, 8 of which have legal recreational cannabis. If a majority of states disagree with federal law… well, the federal law needs to be called into question. And it’s not just states either – the majority of voters think cannabis should be legal.