mysqldump: Does --lock-tables=false work with MyISAM tables?
Using lock-tables=false becomes irrelevant given MyISAM's table locking approach to DML.
You asked

My guessing is all their backups are inconsistent, since they don't lock all the tables, but one by one with every dump. Am I correct?

Yes, you are correct. Furthermore, when it comes to XtraBackup and MySQL Enterprise Backup, there must be a FLUSH TABLES WITH READ LOCK if MyISAM is present.
Oracle and I recently had dialogue. They asked me to change everything to InnoDB in order to use Enterprise Backup. When I explained I had 40GB and 2TB MyISAM (most of the tables had hundreds of partitions), they understood. I still that after all these years, you must still take matters into your own hands for point-in-time consistency with MyISAM.
SUGGESTION
Your best solution would be to use MySQL Replication and run the backups from the Slave
I have suggested this before

Oct 01, 2011 : Quick MySQL Backup
Dec 05, 2011 : Set up a MySQL slave using mysqldump to get the initial data

You may want to use binlog_format=ROW on Master and Slave to catch granular changes before running STOP SLAVE; FLUSH TABLES; on the Slave. Then, you can mysqldump from the Slave without worrying about any Storage Engine locking or consistency issues.
Give it a Try !!!