Can I listen to Stratechery Podcasts in Spotify/Google Podcasts/Stitcher?

No, unfortunately not. Podcasts, like email, are based on open protocols available to anyone. That is why you can use whatever podcast player you wish to listen to most podcasts. Unfortunately Spotify, Google Podcasts, and Stitcher do not support the open protocols that undergird podcasting, so it is impossible to add Stratechery Podcasts.