That chart likely doesn't account for the fairing reuse. The standard fairing halves are now routinely recovered and reused. There is an extended fairing in development for Falcon Heavy, but it may not be reusable, which would reduce the cost difference. And of course, the extended fairing doesn't exist yet.
However, even assuming a reusable extended fairing, preparing a Falcon Heavy involves readying 3 first stage cores in a special configuration for about double the payload (with full reuse). At the moment, throughput is a bigger issue for them than launch costs, and they can likely put up more satellites in a given time period with Falcon 9. They're averaging better than one launch a week this year, and next year might reach two launches a week. That's likely easier to achieve than one Falcon Heavy a week.