Since SpaceX entered the market, ULA has stopped selling both its flight ready rockets with no replacement ready, Ariane has finished flying with no replacement ready, the Space Shuttle has stopped flying and its role has been replaced by SpaceX, Soyuz is no longer available for US and many other markets, China's rockets are similarly unavailable in the US, and a great many new competitors continue not to have vehicles that exist. Electron and some small number of less robust small launch players fly but compete in a market segment SpaceX abandoned in 2009. SLS exists but has zero commercial customers and will never have more. India and Japan's rockets exist but have near-zero market presence.
The strongest sense in which SpaceX is not a monopoly is that other companies let you order flights on vehicles that don't exist yet, but hopefully may soon.
I think you may be reading the term ‘monopoly’ in a nonstandard way. SpaceX being a monopoly just refers to them being a single dominant supplier in active launch, which is a position they built by fairly outcompeting other companies.
The strongest sense in which SpaceX is not a monopoly is that other companies let you order flights on vehicles that don't exist yet, but hopefully may soon.