A distinction without a difference is how I would characterize their point.
There are several ways the “compact” conflicts with the Constitution.
But I don’t see how requiring an Elector to vote for the popular vote candidate does.
Well, if you are referring to the compact, the first thing you have to do is to define "national popular vote".
Like, who counts it? Wolf Blitzer?
Who certifies it? Rachel Maddow?
Suppose it is challenged? Who has custody of the ballots and who supervises a recount?
The whole POINT of the system is to keep all three branches of the national government out of the business of selecting the new President, which is why Bush v. Gore was such a terrible mistake.
And, go further. California, New Jersey, and New York are already devising ways to keep President Trump off the ballot in their states. According to the sole and plenary power their Legislatures have over appointing Electors, this is their perfect right.
Donald Trump got eight million votes in those three states in 2016. Subtract those votes from his 2020 "national popular vote" total, and he cannot win.
Go further. What (in the Constitution) is to prevent California and New York from requiring future candidate certification by the State Office of Social Justice?
There's no way a "national popular vote" compact can work without a Constitutional amendment to provide uniform ballot access and uniform voter qualification rules, which would have to be set by a national body - and after that, you might just as well abolish the States and be done with it.