There are no good sources on this. All best guesses are that TEC's membership have dropped hugely and they're making up the numbers. I never formally renounced membership when I left over a quarter-century ago (maybe I should have, but several years away without a request for a letter dimissory should be grounds for removal) so I wouldn't be surprised if I were still on their rolls.
But it's odd -- the chart you posted seems now to have disappeared.
I
know that we are still on the rolls, despite us notifying them multiple times to take us off. We are still getting mailings from the diocese, our old parish, and a new mission church that was recently started in our area (to replace the orthodox parish that was imploded by the bishop).
I finally called the bishop's office and was informed that you will NOT be taken off the rolls unless you transfer to another Episcopal parish!!! About the only way to get around it is to transfer to the parish of a sympathetic rector, who would then erase you from his rolls and thus cause you to finally vanish from 'the system'.
In other words, everyone who fled TEC to become Lutheran, Catholic, Methodist, Southern Baptist, or Unitarian-Universalist for that matter is still counted as a member!
"How conveeeeenient! Could it be . . . ?"