Quite, curlew. Aside from Toys' relentless use of the word "discourse", which tells you that academia is not far away, their whole argument smacks of an academic enthusiasm for "rational" solutions that they can get a paper out of ahead of pragmatic solutions that work.
Using FSM to target PP is a broadbrush solution, and no-one is denying that. However, it has the advantage of delivering resources ahead of the problems, rather than - as you point out - afterwards. Dealing with academic failure is massively, massively more expensive than stopping it from happening in the first place, and if the effect of tying PP to FSM is to splurge resources to those that don't actually need it, then unless that splurging is factors of three or more it's still much cheaper than allowing problems to develop and then trying to fix them.
Identifying "academic underperformance" or whatever their phrase is will be very hard. It means more testing, more reporting, more standardised assessment. It means nuanced judgements about expectations, and it's almost inevitable that such a system will only deliver resources long after the problems have become embedded. For children not in receipt of FSM who fall behind there are other mechanisms, (SA, SA+, Statements, IEPs, etc, etc, etc) and in some cases resources follow those interventions. But it's too late.
PP is a noble attempt to take money from the leafy suburbs and pump it into schools that can make a difference. If people don't like it, the onus is on them to show why their proposal is better. And vague wittering about "discourses" and, I suspect, "narratives" isn't good enough.