It's an ambiguously worded sentence. What they mean is: if you take the entire Japanese desktop computer market (not some subset of it, like "low-power desktop

It's an ambiguously worded sentence. What they mean is: if you take the entire Japanese desktop computer market (not some subset of it, like "low-power desktops", the "entire" desktop market), and you look at which individual computer manufacturer has the highest market share, Apple has just overtaken Lenovo Japan. How I might've worded it: Apple now has a marketshare plurality in the Japanese desktop computer market after the introduction of their M1 Mac Mini.