How do the wicked Thamud swear by Allah, though they are disbelievers, in Surat An-Naml, verses 48-49?

Disbelievers may possess some faith in God, especially concerning matters of Lordship. Thus, when asked about their Creator, they say, "Allah," as per His statement, "And if you ask them, 'Who created them?' they will surely say, 'Allah.' So how are they deluded?" For this reason, their swearing by Allah is not surprising.