It sounds like you're mixing name limitations up a little bit. Computer name limitations, domain name limitations, and SamAccount name limitations are different things altogether.
The SamAccount name limitation is 20 characters.
The NetBIOS name limitation (single label computer name) is 15 characters.
For info on DNS and FQDN name limitations, see here:
Disabling NetBIOS has nothing to do with the NetBIOS computer name. Every computer object has a NetBIOS computer name regardless of whether or not NetBIOS is enabled.