Zhang Yunpeng was indeed quite sincere; if Zhou Guangxiang were to join the Yellow Sand Sect, he would be willing to take him as his first disciple and cultivate him with all his effort.
As he stated, if Zhou Guangxiang joins the Yellow Sand Sect and acknowledges him as his mentor, with the support of a Purple Mansion Stage master like him, coupled with the full help from the three Foundation Establishment Cultivators of the Zhou Family, his cultivation path would be smooth, and the chances of establishing a Purple Mansion would undoubtedly be much higher than staying with the Zhou Family.
However, no matter how eloquently Zhang Yunpeng spoke, Zhouyang knew that once Zhou Guangxiang joined the Yellow Sand Sect, even if he eventually became a Purple Mansion Stage Cultivator, he would be a Purple Mansion Cultivator of the Yellow Sand Sect, not one of the Zhou Family.