Well you do have a point. It would be absurd. Just like the opposite. Both decolonizing and "further" colonizing maths makes no sense and is a waste of time at best...

Is colonizing now a synonym for cultural fusion?

Surely the arabs have been colonizing mathematics by translating the indigenous greek works

The arabs of the Abbasid Caliphate braided a rope by unifying Greek, Babylonian, and Indian mathematical and scientific works after translating original works into Arabic and extending them.

As well as everyone else, why is it colonizing when the west do it to arabic mathematics and not when the arabs do it to western mathematics?

That argument is rather weird as mathematics was never about culture, but rather about logical truth

Since my example was apparently poorly chosen due to my own ignorance, and you're finding it worthwhile to have this discussion, I'll conclude that studying mathematical history ("decolonizing mathematics") is useful.