Not sure that I agree with that statement.
From Universal Lasers:
Cast acrylic is ideal for laser engraving because it creates a frosted surface. … Extruded acrylic is ideal for laser cutting because it forms a flame polished edge during the laser cutting process.
From Epilog Lasers:
There are actually two types of acrylics suitable for use with the laser: cast and extruded.
Cast acrylic sheets are made from a liquid acrylic that is poured into molds that can be set into various shapes and sizes. This is the type of acrylic used for most of the awards you see on the market. Cast acrylic is ideal for engraving because it turns a frosty white color when engraved. Cast acrylic can be cut with a laser, but it will not result in flame-polished edges. This acrylic material is better suited for engraving.
The other type of acrylic is known as extruded acrylic , which is a very popular cutting material. Extruded acrylic is formed through a higher-volume manufacturing technique, so it is typically less expensive than cast, and it reacts very differently with the laser beam. Extruded acrylic will cut cleanly and smoothly and will have a flame-polished edge when laser cut. But when it is engraved, instead of a frosted look you will have a clear engraving.
From Trotec Lasers:
Cast and extruded acrylic adapts very well to laser technology and both are an ideal material for laser processing. It is beneficial to have a good understanding of the differences when selecting the right acrylic for your application, because each type will behave differently to laser processing.