Barbarian magic suppression skills...

If he had Anti-Summon, he could bombard him with various mass Darkness spells - in my opinion, that would be much more effective!

This is effective when there is "meat" to attack, which the Warlock simply lacks (well, except for Hydras, of course). There wouldn't be a Stopper who could gather on himself, albeit weakened and slowed, but still a formidable army of the Stronghold. The Phoenix fulfilled this role.
In general, of course, you can argue for a long time about what is better, but the numbers clearly show that magic schools based on developed Witchcraft are weakened more effectively. And these are Chaos and Summoning. Light and Darkness are weakened less effectively.