How do I use the computed twig element to get the value from a custom composite element?
I suppose I can answer my own question.
It didn't work because I didn't use the delta. For this to work, I need to do this instead
{% for data.products as p %}
{{ p.product }}
{% endfor %}

This is because we are using a custom composite element where the products are stored like
$products[0]['product']
$products[1][product']
...

I missed the array key and hence didn't get any value to the element in the custom composite.
FYI: The answer was in the 'Help' section of the module already, but couldn't really understand it well at the time.