I can’t read other people’s minds, but I would interpret that advice as being focus on your core flying skills / CRM / decision making rather than which particular sim you will be assessed on.
You could go and find a 787 sim and practise some raw data on that etc. but I suspect it wouldn’t make that much difference what Boeing type you elected to do any practise on. Besides the sim used for assessment might be changed at short notice..
If you’re currently flying the line you could do some of your ‘practise’ at work.