Thank you, it works!
I also had to change the AS numbers to strings (i.e. '3215' instead of
3215`):
name: zz-whitelist-AS
description: Whitelist some AS
debug: true
whitelist:
reason: Whitelisted AS
expression:
- evt.Overflow.Alert.Source.AsNumber in ['3215', '15557', '12322', '5410']