That's the revelation recently released by another linguist and defender of the authenticity of the graffiti, Juan Martin Elexpuru: that Txillardegi believed them to be true.
José Luis Álvarez Emparantza, best known as Txillardegi, who died in January this year, is considered one of the most influential Basque linguists ever.
According to Elexpuru, he wrote to him a letter in December 2009 in which he stated that he had the Iruña-Veleia graffiti as authentic and that he considered Hector Iglesias' paper Les Inscriptions de Veleia-Iruñea to be outstanding (bikaina).
Interestingly he considers the inscription Yaveh zutan izana as an archaic form of hika (roughly equivalent to using the pronoun thou), mentioning that Johannes Leizarraga in the 17th century wrote aiz for (modern) haiz (thou art). Hence for Txillardegi this sentence should be read (in modern Basque) as Yaveh zerutan haizena (Yaveh who art in heaven).
The issue of zerutan (modernly would be zeruan, sing., or zeruetan, pl.) he thinks it may be related to dialectal variants like surtan (from su: fire).
In another letter he lamented that he could not, being in his 80s, anymore get involved in this affair, as he would have no doubt have done if he was younger.