Distribute is the key word. In general, companies can fork GPL code to power the backend of public or internal services, without being required to publish their

Distribute is the key word. In general, companies can fork GPL code to power the backend of public or internal services, without being required to publish their modifications, but once they distribute the software (allowing binaries/scripts to be downloaded, including it in app, embedding it in firmware...), then they must open up the source.