No, if they wanted to pronounce it with a K they could easily pronounce it maku or maaku. They pronounce it maha because in Gaelic the CH is a voiceless velar fricative /x/ (like the CH in the Scottish Loch Lomond, German buch, Greek τέχνη techni, Hebrew מִיכָאֵל Micha'el, etc) and the glottal fricative /h/ is the closest sound in Japanese phonology. If an English speaker were to pronounce it Ma-h with a bit of breathiness at the end, it would be closer to the right sound than /mak/.
Nevermind that the Gaelic MH is pronounced [v] tho....
