It's true to some extent. There is undeniable evidence that funding comes from there, especially KSA but it's hard to say whether it's the govt doing it or the citizens themselves. So it might not be their official stance. Recently a kind of spat took place between Pakistan's foreign office and KSA over this exact same issue.
A Pakistani minister said KSA was funding terrorists, and KSA spokesman replied that
all KSA funding goes through Pakistani govt and therefore is legal, to which our foreign office replied that
only official donations go through Pakistani govt, but there are still other donations that are not scrutinized (kind of rejecting their statement).
Saudi Arabia denies funding 'extremist mindset' in Pakistan - Pakistan - DAWN.COM