Account.Name
is not an external Id field! And neither is AccountNumber
for that matter. I guess you're confused because it can be used as match field for de-duplication in "import my accounts & contacts" etc?
Here are my results, I don't think you'll get different ones:
Schema.DescribeFieldResult f = Account.Name.getDescribe();
System.debug(f.isExternalID()); // false
System.debug(f.isUnique()); // false
System.debug(f.isIdLookup()); // false
You'll need to create your own Ext. Id. field if you want to use it in upserts (and fill it with data). In my org we have custom Account_Number__c
...