You've effectively changed what he said. He said something much simpler: there are children that use Tumblr; because there are children that use Tumblr, removing porn from Tumblr should not be controversial.
A "nuanced" reply would assess the risk of exposing children to the fascinations of various alternative communities that do not dabble in child porn. I don't see "Porn is obviously bad", but I do see exposing children to porn is obviously bad.
As it stands, Tumblr is not just banning someone's stash of hardcore sex videos. It also is banning, say, a casual vacation photo taken on a beach where topless sunbathing is acceptable (due to the explicit ban on female nipples except for a few narrow contexts). On a more "adult" level, it is also banning stuff in between the two categories, like say boudoir photography (which is usually adult in nature, sometimes erotic / titillating, but is usually not explicitly pornographic.)
Even the "any content that depicts sex acts" seems vague. Would something like, say, Rodin's "The Kiss" -- https://en.wikipedia.org/wiki/The_Kiss_(Rodin_sculpture) -- be flagged? I don't know, it depends on what the auto-algorithms sees as a "sex act" I guess. Certainly if you were an artist that dabbled in any sort of erotic drawing / painting, the vague definition would probably be a worry IMHO.
(Tumblr is already 13+ already. Commercial crass porn is one thing, but I personally don't think there is any problem showing a 13+ year old this Rodin sculpture or a casual beach shot where topless sunbathing is acceptable.)
As a parent, the idea of exposing my kids to the internet is frightening. There are so many paths by which they can get exposed to stuff they shouldn't be exposed to. I, personally, think that attempts to make more of the internet child friendly are good. The internet as it exists today is like walking down the Main Street of some town, except that people surprise you by randomly jumping out from behind mailboxes and shrubs and engaging in hardcore sex on the sidewalk. It's insane.