Arduino is supporting the official MicroPython project and has released ports for a number of boards in addition to tools like a simple MicroPython IDE and a GUI installer.
True, which is to say they are a competitor with alternate toolchains
If I get a board with an Arm0 chip and want to flash it with micropython, Arduino is not the only game in town (not even my first choice by a long shot, ymmv)
With the lower power AVR chips, Arduino's only competitor was the project it was forked from, Wiring - any other embedded toolchains were not even trying to reach the casual hobbyist, the Arduino IDE dominates that niche.