The essence of centrism is an acceptance of a limit for everything. This means there are limits in all directions. The image of this is a closed convex curve with a center in the middle of the region enclosed.
Without limits, there is no center. A center is always within limits. If there is any direction without a limit, the curve is not closed and there is no center.
Non-centrists are extremists in at least one way. They reject a limit in at least one direction. They are not only not in the center, but they reject the existence of a common center.
The slogan “No enemies on the Left” is a left-wing motto that goes back at least to the 1930s. It reflects an attitude that in the direction of leftist politics, there is no limit. Because it lacks a limit in at least one direction, it is extremist in at least one direction.
Most political groups promote some cause or idea that takes precedence over all other causes or ideas. They may hold these in a limited way, but unless they have ways of limiting the range of their support, they will tend to go further and further in that direction. They are or will become extremists.