I maintain the nest integration, and have been preserving the legacy integration (e.g. adding tests to keep it working as the sdm API integration evolves). The original author isn’t on the project.
Nothing has changed in the legacy integration code base.
Googling this, it seems like the API is just rate limiting you:
But I have no idea really.
I would recommend upgrading to SDM as in the next release I marked the legacy nest integration is deprecated and giving folks a few months to move off. (Upgrade, or move the code to a custom component, whatever)