That's all theoretical anyway. I'm not on GMail; I technically have this feature in the e-mail client on my phone, but I never even use it.
EDIT: The article mentions those summaries would apply to email threads too. I think that could be helpful. I've got tons of threads from some mailing lists that tend to grow large (10-20+ messages); catching up with them or revisiting old ones is tedious; an AI TL;DR of such threads actually sounds useful.
Also I'd love for e-mail clients to have a feature reminding normies when they've "forgotten" to address some questions from the sender.