Django's mark_for_escaping doesn't work with custom template tags?

Question

I have a Django template tag that looks like this:

from django import template
from django.utils.safestring import mark_for_escaping

register = template.Library()

@register.simple_tag(takes_context=True)
def hello_world(context):
    return mark_for_escaping("Hello, <world>!")

But when it's used in templates the output isn't being escaped.

What's going on? Is the output of template tags always assumed to be safe?