As of mid-December, Windows Insiders with ARM64 PCs were able to run x64 apps in emulation, opening up the PCs to the rest of the Windows ecosystem. Yesterday, however, the team released a cumulative update for the latest Dev channel build, and as it turns out, the update breaks the feature, so you might want to skip it.
The update in question is KB4601937, and the blog post has been updated to day, "After installing Build 21292.1010 (KB4601937), x64 emulation on ARM PCs will not work. There is an issue with the way the update is installed that breaks this functionality. As a workaround, you can uninstall KB4601937 to get x64 emulation working again on your ARM PC."
This cumulative update was introduced as something that includes nothing that's new. According to Microsoft, it only exists to test out the servicing pipeline, meaning that the company just wants to make sure, as it does periodically, that it hasn't broken the ability to ship cumulative updates. But being an update with no features, it would be reasonable to expect that it's low-risk, and that it wouldn't break.
The update should be easy enough to roll back, if you're experiencing issues. From Windows Update, go to 'View update history', and select 'Uninstall updates'.