The issues of using classroom assistants to lighten teachers' load and using them to replace supply teachers seem quite separate and certainly the first cannot be used to justify the second.
As a supply teacher and a student struggling with a PhD on classroom misbehaviour, I found this insulting, rather depressing and certainly unhelpful. If Sir Bob has had bad experiences in his past with supply teachers it might be more helpful if he were to consider how he might have alleviated these bad experiences by being more understanding both of the particular difficulties which supply teachers face and more appreciative of the particular classroom interpersonal skills which supply teachers who are properly supported come to develop.
Even educational researchers produce mediocre or, as some might argue, useless research but that does not make them mediocre or useless people. Sir Bob will do little to help communication between practising teachers and researchers if he is happy to risk alienating a 10th of the former with a few ill-chosen words.
Petworth, West Sussex