In this case the author is talking about using the 14th amendment to strike down state laws, but I was just talking about federal law. E.g. if the federal government allows people in DC to do something, then it has to allow everyone else to do that thing also.

The author is right that strictly speaking the equal protection clause only applies to state laws, but the supreme court has also retroactively applied it to federal laws as well:

https://en.wikipedia.org/wiki/Bolling_v._Sharpe

The parent post was arguing that it would be hard to prosecute marijuana possession under federal law because federal law allows possession in DC. The link argues that state laws against marijuana are not affected by federal law. So in Virginia, you won't be able to escape prosecution for marijuana possession under Virginia's law just because it is not illegal under federal law or another state's law.

You are not missing anything. The link does make the exact opposite point: "you will not be able to argue that you should be protected by the Constitution."