Rather than passing them through to the application. Invalid STUN
packets are still passed through, but it causes unnecessary noise and
corruption for higher-level applications to receive STUN packets they
weren’t expecting.

This is permitted by RFC 5389, §7.3.0:
    If any errors are detected, the message is silently discarded.
    In the case when STUN is being multiplexed with another protocol,
    an error may indicate that this is not really a STUN message; in
    this case, the agent should try to parse the message as a different
    protocol.
where I interpret ‘error’ to mean ‘validation error’ rather than (e.g.)
‘unrecognised attribute’ or ‘unmatched response’ where the STUN packet
is otherwise perfectly formed.