Not to be that guy, but the article discusses this at some length. Find this bit:

> However, such tests rely on introspection and self-reported experience, which made some neuroscientists doubt that aphantasia was real. Could reported differences in visual imagery be a language disconnect, given the ambiguity in how we describe our inner worlds? “It could be the case that we’re all actually experiencing the exact same apple, we’re just describing it differently,” said Rebecca Keogh, a research fellow in cognitive neuroscience...

The following ~8 paragraphs discuss the evidence for it beyond subjective reports.