That is because Allah alone is the Truth, and indeed whatever they are invoking besides Him is falsehood, and indeed Allah is the Highest, the Great.
‘The Truth’ is mentioned for both Allah and His Book Adhikr. It is told in verse 17: 81 that Truth has arrived, and falsehood has perished; indeed falsehood has to perish. Whoever invokes others besides the Lord, they are invoking the creatures like them as told in verse 4: 117. To hold Allah is to hold Adhikr (Truth). Whoever seeks Guidance anywhere other than the Guidance Adhikr, is in obvious astray. Through verse 20: 14 it is commanded that you establish prayer to keep up Dhikree (Adhikr). Since the Arabic Quran reading Fujjar are hiding and rejecting Adhikr, they will get Hellfire as a fine for performing prayer, fasting, Hajj, Umra, etc. as explained in verses 2: 186 and 25: 65-66. See explanation 7: 194-195, and 22: 71-73.