SWQL Studio has a hard-coded list of keywords to highlight. ISNULL isn't in the list, though it should be. Don't read too much into it. ISNULL works fine with SWIS.
"ISA" is a keyword that gets highlighted because it is in SWQL Studio's list. This is an operator in SWQL that doesn't really have a direct equivalent in SQL. It's a boolean operation on types. It tells you whether the left operand is the same as or a subtype (direct or indirect) of the right operand. Example usage:
SELECT FullName FROM Metadata.Entity WHERE FullName ISA 'System.ManagedEntity'