Rashi, Targum Yonasan: It is Har ha'Zeisim, i.e. Har ha'Mishchah. Since it was mentioned due to idolatry, its name was switched to ignominy.

Radak: They destroyed the idolatry, but not the Bamos, for people offered on them to Hashem. It says about all the kings "veha'Bamos Lo Saru Od ha'Am Mezabchim u'Mekatrim ba'Bamos" ¹ Yoshiyah destroyed all of them, since initially they were made for idolatry, or so people will offer on them even to Shamayim. Bamos were forbidden after the Beis ha'Mikdash was built. This is why it says "there was no king before him who repented to Hashem with all his heart