In this experiment, the product has color, it is easy to measure its through optical means - therefore we are measuring the appearance of product.
In a typical experiment, we do not know how much enzyme is needed, so if we want $ V_{max} $, we simply have a lot of substrate - then enzyme will most likely be the limiting factor.
Therefore the answer is:
In this experiment, the enzyme velocity is measured by the appearance of the enzyme's product.
In a typical experiment, the substrate is in excess, and the velocity will be measured quickly after initiating the experiment.
There is a maximum velocity because as substrate concentration increases, availability of enzyme is going to be the limiting factor, that's why the answer is:
At high substrate concentrations, the enzyme is saturated and cannot catalyze any faster.
Without enzyme - the reaction is practically not happening, so that rate increase due to substrate concentration can simply be ignored.
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