What are the App Store fees for subscription to an IoT hardware product?
Quoting the text from App Store Review Guideline verbatim:


  3.1.4 Hardware-Specific Content: In limited circumstances, such as when features are dependent upon specific hardware to function, the app may unlock that functionality without using in-app purchase (e.g. an astronomy app that adds features when synced with a telescope). App features that work in combination with an approved physical product (such as a toy) on an optional basis may unlock functionality without using in-app purchase, provided that an in-app purchase option is available as well. You may not, however, require users to purchase unrelated products or engage in advertising or marketing activities to unlock app functionality.


From your question:


  When the app provides the UI for an external IoT device, and there is an annual fee charged to the user (which may be via an external link) does Apple charge a percentage?


How are you charging the fees from the user? If you are charging it via In-App purchase or recurring subscription, Apple takes its cut. If you are charging the user via external means and your app doesn't violate any App Store guideline, Apple won't charge anything.


  My reading of this line is that no fee is charged.


Your understanding is right and no fees will be charged.

In case of Nest Aware (I have never used the app), if the user buys the subscription via In-App purchase made in iOS, Apple will take it's cut.