Verifiable sources do not ensure truth. (It's not clear to me that there's any way to ensure truth... you know, observer A says it's a wave, observer B says it's a particle... blind men... elephant... etc.) What verifiable sources do is to enable the reader to _make a judgement_ of the probable truth of the fact. That's why it's much more important to have _a_ source, _any_ source, than to have a reliable source. As long as there's some kind of source _the reader can judge its reliability._ Of course, the better the source, the more readers will judge that the fact is probably accurate, so the more reliable the source, the better.