Not really sure your reasoning is valid either.
Firstly it's based on the premise that being disabled means you are less productive than an able-bodied counterpart - presumably because you're disabled. It doesn't take into account job type, the spectrum of disability, or the amount of disabled people in employment period.
On that final point - according to scope, 3.4 million disabled people are in work, which amounts to approximately 30% of all disabled people in the UK regardless of age (
https://www.scope.org.uk/media/disability-facts-figures)
Also - I can't imagine that disabled employment has surged in recent years?
Common sense would suggest that it would have largely stayed constant, given so few of the work force are disabled, and so few of the population are disabled as well - that number is unlikely to offset the fact that mostly everyone is working longer hours, and being less productive because we're struggling.
If his logic is correct then productivity will plummet when a large amount of the disabled population lose their benefits and are unable to work?