Your new tree must be magnificent. I looked up cambistat and read over the description of the tree's location. My comments are below the information about chemical.

Cambistat improves tree health by reducing the amount of energy allocated to canopy growth. This energy  can then be used for other processes such as fibrous root development and defense chemicals. This makes Cambistat an ideal tool for complementing other tree healthcare practices and treatments. Cambistat has been shown to reduce growth, and its active ingredient has been shown to increase a tree’s ability to withstand drought conditions and the effects of urban tree stress.

The Science

The active ingredient in Cambistat and Trimtect is a molecule called paclobutrazol. The molecule is part of a highly active group of plant growth regulators and fungicides known as triazoles. Widely used for decades for tree care and in vegetation management, paclobutrazol consistently reduces the shoot growth of plants by 30-70% during the growing season.

How It Works

When applied, a lone electron on a nitrogen atom reacts with the enzyme kaurene oxidase. This interaction blocks a key step in the formation of gibberellins, resulting in a reduction of vegetative growth.

The critters living in the tree will not be harmed. Plant growth regulators do not harm insects and mammals except in high doses.

From the description of the tree's location the tree company is recommending managing the growth of the tree and keeping it in bounds its location is a problem. Too much concrete and too close to a house means even a slow growing oak is going to out grow its location eventually.