From https://gerrit.wikimedia.org/r/#/c/215646/,

The selser failure for "HTML headers vs TOC" is kind of interesting.
In the src wikitext, the behavior switch and heading have a nl sep.
However, it's edited away by the random change. The wt2wt sees two
old elts on the same line and is happy to serialize them as is.
Selser, for its part, looks for the original separator and
reinserts the nl.

Try with,

__TOC__
=== hi ===

Parse it, remove the newline, then pass it to node parse --selser --oldtextfile ... --oldhtmlfile ...