I agree it's a very odd and unclear way of expressing it. I'd think it's that the author estimates that people in a family that size and who are on benefits would need to spend 63p in every £ they receive to meet basic food and household energy bills. However, because they can't possibly afford that, it's assumed that they have to skimp on those despite any possible impact on their health.
The organisation hasn't published the report yet so I can't work out what the writers have done to make their calculations. The journalist hasn't made it that clear. And later on, they refer to the "poorest families", but it's not clear if they are people without anyone in employment or without anyone who is eligible for disability benefits, so they are subject to the benefits cap? (iirc, people who are in work, and in receipt of benefits, shouldn't be subject to the benefits cap.)
I wish they'd given some examples to make it clearer.
I know that if you were to remove our mortgage payment, council tax*, insurance, commuting etc. costs from our post-tax income and then estimate our energy costs, they'd be a very different % than our our net post-tax income. E.g., let's say, energy bills for our home are 10% of our net monthly combined income and the food was similar at around 10% so we'd be paying 20% of our net income.
However, the energy bills would be around 22% of what's left after paying housing etc. and the same for food, so we'd be paying about 44%.
*I think CT reductions vary, but I'm ignoring that to simplify the calculation.