Why were the believers mentioned in verse 35 of Surat Adh-Dhariyat, and the Muslims mentioned in verse 36, and does this mean that they are Muslims and not believers?

Al-Qurtubi mentioned in his Tafsir that Allah's saying: "Then We brought forth such as were therein of the believers" means that Allah brought forth the believers before the destruction of the people of Lut. As for His saying: "But We found not therein any but a single house of those who surrendered [to Allah]," it refers to Lut and his two daughters. He called them believers because every believer is a Muslim. This does not mean that they were only Muslims and not believers; rather, they combined both attributes.