Earlier quoted context omitted.
Nor do custom tooltips though because the majority of websites and apps fire them on a mouseover event.
Tooltips on mobile are tough. Tooltips usually are on links. For example here on HN, hover over "11 minutes ago" in the comment header and it shows the time. Could this be made available to mobile users? Seems tricky. My first thought would be to show it when the user taps and holds. But that also triggers the browsers default context menu for links. So it would make a murky experience.
The first doesn't exclude the second. That's the way how https://xkcd.com/ is doing it. It just shows both.