Relative concentration of enzyme vs reaction product
Remember that the concentrations you report are in mass units rather than mole units. If the protein has a mass of 40,000 Da, reasonable for many proteins, then 20 mg/liter means $0.5$ x $10^{-6}$ mol/liter. A typical small-mass product of an enzyme reaction might have a mass about 200, so its apparently lower mass concentration of 2 mg/liter is $10$ x $10^{-6}$ mol/liter, 20 times higher than the enzyme concentration in terms of numbers of molecules per liter. The mole or molecule ratios are what you should think about in terms of catalytic function.

Furthermore, from what you provide we don't know whether the enzyme was produced under circumstances that allowed it to express its activity; there may have been limitations on substrate availability, presence of inhibitors that would be removed in later purification, and so forth. In this scenario, evidently to obtain enzyme for downstream use, the emphasis is typically on getting large amounts of the specific enzyme regardless of whether some reaction product is formed. I don't find the enzyme concentration particularly high for something that the investigators are trying to over-express as in this case.

The Bradford reaction is a mainstay of standard biochemistry, as it's simple and straightforward. Different proteins can bind the reagent to somewhat different extents, so there is a second-order issue of whether the protein used for the standard curve and that being measured are similar enough in that respect. In practice, what's important is to provide enough information in the methods that are described so that someone could repeat the work to determine the relation between the Bradford measurement and some other more accurate method for this particular protein.