It says that most roads in urban areas tend to fill up to a certain rate of traffic regardless of the absolute capacity.
With Loughborough Rd closed off, CHL will fill up to a certain rate of traffic.
With Loughborough Rd open, both CHL and Loughbrough Rd will fill up to a certain, similar rate of traffic.
In either scenario, a sudden increase in traffic demand will cause an issue.
Of course, you can argue that having both roads open would allow that sudden increase to be shared between them which is true. The greater the overall capacity of the road network, the less effect sudden increases will have. But following that logic could end up with us building that motorway through LJ that thankfully never happened, and more traffic generally which would mean even more pressure on smaller streets. So, a better solution surely is to try and be a bit clever about managing deliveries to big construction sites to reduce their impact. Consolidate deliveries, and avoid making them during rush hours, for example. This is why planning permissions for large sites require logistics plans for deliveries, and can set restrictions on delievery times etc.
Construction Logistics Plans - Transport for London
It doesn't help that lorries are banned from most roads in London between 2100 and 0700. When you combine that with avoiding rush hours and the fact that HGV drivers can only drive for a limited time before they must take a break, you have a logistics nightmare.