AusBonsai

I have some comparative photos of what I have labelled as each - the sieberi are a fair bit older, but you can see the difference in leaf size and habit.
C. pityoides - smaller leaves, thinner and slower growing. Not as thick a bark, possibly.
Another - same have quite interesting roots.
C. sieberi = Melaleuca paludicola
and another. They can grow quite thickly and dense, with good taper.
A small C. pityoides in front of a C. sieberi for comparison. Both good fun.

It was explained to me by a patient botanist that there is a wider range of difference between the extremes of Callistemon and the extremes of Melaleuca individually, than there is between the Genuses themselves. And the genetic analysis shows that they are more closely related than appearances would suggest. It's all beyond me. I wish they'd make up their minds.

Gavin

I'm pretty sure you know this, Gavin, but here goes anyway.

A couple of years ago, I posted a summary of Ausbonsai posts on callistemons. Here is a link to that post:
viewtopic.php?f=55&t=29478. The post began by addressing the question of what makes callistemon and melaleuca different. For ease of reference, I have copied that text into this post (see below, in italics). It is possible that some of the information comes from the same patient botanist you refer to.

Melaleuca and Callistemon: Why are they Different?
First, I think it is necessary to deal with the debate as to whether or not Melaleuca and Callistemon belong to the same genus.
The Australian National Botanical Gardens answers the question What is a Bottlebrush? this way: “Bottlebrushes are members of the genus Callistemon and belong to the family Myrtaceae. They are closely related to paperbark melaleucas, which also have 'bottlebrush' shaped flower spikes. It is difficult to tell to which genus some species belong. Botanists are currently closely studying these plants to determine how they are best classified. There are 40 species currently called Callistemon” and some 300 species called Melaleuca.

The Australian Native Plants Society (Australia) has an article on its website (http://anpsa.org.au/mel-cal.html) that explains the technical difference between the genera. The main difference has to do with the way in which the stamens (the male parts of the flowers) join to the floral tube.
The current (2021) situation is that State/Territory herbaria in the Northern Territory, Queensland, Australian Capital Territory and Tasmania accept the joining the genera, while herbaria in the other States do not at this stage.
Now for the summary of AusBonsai posts.