The filter should be written in the following form.
<$action-navigate $to={{{ [subfilter{$:/config/RandomSelection/Filter}random[1]] }}} />
or
<$action-navigate $to={{{ [subfilter{$:/config/RandomSelection/Filter}] +[random[1]] }}} />
He may be using some plugins or custom tiddlers.
My explanation may not be quite right. But the filter is actually dealing with text references. Thus a filter symbol handles a quoted text. There are three ways to make a citation.
<> is usually some variable or [tag<currentTiddler>].
[] is usually the name of some specific entry. Like [tag[HelloThere]]
{} is usually the text field value of some tiddlers, but of course other field values can be referenced by something like [tag{HelloThere!!caption}].