I m confused it seems to be possible to
I'm confused, it seems to be possible to either purge by key locally or purge all assets globally. Is it possible to purge by key globally?
Currently, it is not possible to purge a URL stored through Cache API that uses a custom cache key set by a Worker.Your best bet is global purge but you can do by hostname

To purge an asset globally, you must use the standard cache purge options. Based on cache API implementation, not all cache purge endpoints function for purging assets stored by the Cache API.
All assets on a zone can be purged by using the Purge Everything cache operation. This purge will remove all assets associated with a Cloudflare zone from cache in all data centers regardless of the method set.
Available to Enterprise Customers, Cache Tags can be added to requests dynamically in a Worker by calling response.headers.append() and appending Cache-Tag values dynamically to that request. Once set, those tags can be used to selectively purge assets from cache without invalidating all cached assets on a zone.
Currently, it is not possible to purge a URL stored through Cache API that uses a custom cache key set by a Worker
cache.deletehttps://api.cloudflare.com/client/v4/zones/ZONE_ID/purge_cachehttps://api.cloudflare.com/client/v4/zones/ZONE_ID/purge_cache