OP, children on the autistic spectrum are not the only ones who can have meltdowns as opposed to tantrums: the same can apply to children who have been traumatised (e.g. adoption trauma) and/or have various MH issues (not necessarily diagnosed at the time).
I'd say it's a meltdown if the child:
*cannot understand what is being said to them
*cannot recognise their nearest and dearest
*is unable to stop even if offered the thing that triggered the meltdown
*even though of an age to understand their actions, will persist in inflicting significant harm on themselves without intending to do so
my db (adoption trauma) once kicked a deep hole in a wooden door while barefoot
my dd (MH issues later diagnosed, now more or less functioning on medication as an adult) used to be completely out of it, destroyed her favourite possessions, bit me so I still have a scar 15 years later, had to be restrained to avoid causing herself serious harm
despite never having been rewarded or having had a rule relaxed over an outburst, both db and dd kept having these what-ever-I-may-be-allowed-to-call-them until the age of 10; dd then went on to self-harming and later to suicide attempts
neither of them had sensory issues, but if you insist that using the term meltdown is hi-jacking something from your children, then we need a new term, because whatever it is, it is bloody not the same as a naughty child thinking mummy will give in and buy him a sweetie