The person speaking utilized the Soul-Ensnaring Technique of Enchantment Valley. Those demon cultivators with low morale were influenced by his words and involuntarily believed what he said.
This doesn't mean that the speaker's strength is truly that formidable to affect so many cultivators.
Rather, deep down, these demon cultivators did not want to believe Wei Yong's words. When guided by his "logical and justified" rhetoric, they deceived themselves into believing what he said.
However, even though this method helped rally their own side's morale, within the hearts of the demon cultivators in Spring Sun Valley at the Golden Core Stage, they understood that what Wei Yong said was most likely all true!
Otherwise, unless the higher-ups of the Heavenly Spirit Legion had lost their minds, they would never launch a full-scale assault so directly.