Public trackers do use DHT.

DHT enabled in options

Question: if public trackers don’t use DHT, where do peers with H flag come from?

From the DHT network. As you said you have DHT enabled in your options.

In other words public torrents find peers via DHT/PEX as well as any trackers you may have on them. In fact a public torrent doesn't actually need any trackers on it to function, it'll still find peers via DHT/PEX.