When religions like Buddhism are non-Vedic, how can Veda be the source of all knowledge?

Prabhuji could you please explain what do we mean by Vedas mean All knowledge, while we say for example Buddhism, Jainism, Charvakism etc are considered atheistic from vedic perspective since they dont accept vedas. So if there is knowledge not part of Vedas then how does Vedas mean ALL knowledge.