Horizontal progression could be seen as on building upon something that already exist. This does not mean it is not innovative, as successful horizontal progression can still be very lucrative, complex and rewarding.
Creating competitors of an already existing market, for example a new video platform, could be seen as horizontal progression.
Reference
Thiel, Peter, and Blake Masters. Zero to One: Notes on Startups, or How to Build the Future. Virgin Books, 2014.
Highlights or timestamps
This is known as horizontal progress. It’s based on expanding on existing ideas and innovations.
— ^337a6a from Zero to One