Life is NOT, in fact, meaningless without God. I do not believe in believe in God (as WLC uses the term) and yet my life is chocked full of meaning.

https://en.wikipedia.org/wiki/Misleading_vividness
(also known as anecdotal fallacy)

Meaninglessness is a valid possibility.

conceptually valid perhaps, but not scientifically:
a) because it precludes science/knowledge. To say something is unknowable (i.e. 'no purpose') is anti-science.
b) meaninglessness has never been observed by science - everything is cause and effect.

The notion that existence is meaningless is essentially a superstition (belief in the supernatural) - it's outside the laws of nature in which everything has purpose/reason for existence - as defined by the law of cause and effect - which is the foundation of all science. Cause is reason, reason is purpose, purpose is meaningful...