You cannot make a field to field comparison in
SOQL i.e. as mentioned in the question, you cannot compare
X.Parent.AccountId != X.AccountId.
There is other workaround by which you can do this i.e. by creating a new formula field which evaluates to
X.Parent.AccountId = X.AccountId and
false when it is not same.
The formula for your new field would be
IF(X.Parent.AccountId = X.AccountId, 'true', 'false')
And then you can query for records where this new formula field is 'false'
So your query would become
SELECT AccountId,Id,ParentId,Parent.AccountId,RecordTypeId,RootAssetId FROM Asset WHERE ParentId != NULL AND New_Formula_Field__c='false' AND RecordType.DeveloperName='RecTypeABC' limit 10
You can refer Field to field comparison in WHERE clause of SOQL for further help on this topic.