It is true that fluctuating electromagnetic fields inside and outside of two plates will cause the plates to attract each other. It is also true that upon collision, some amount of thermal energy can be potentially generated. However, it is equally important to remember that work had to be done in order to separate the plates in the first place. If you wanted to repeat this process in a "heat engine" type of way, you would have to do work against the Casimir force to separate the plates again, and this work in the ideal case would be equal to the energy gained from moving the plates.
In this light, one cannot extract electromagnetic zero point energy for free. While there is strong evidence for the presence of zero-point fluctuations which have physical consequences (i.e. the Casimir force), this zero point energy only acts as a reservoir which can mediate interactions, but cannot act as an infinite source of energy.
This does not mean, however, that zero-point fluctuations are not interesting and potentially useful! A related effect that was speculated about in the 1970's is the so-called "Dynamical Casimir Effect." In its most conceptually simple form, two plates are placed near each other, and then they are oscillated back and forth very quickly (at frequencies comparable to the resonance frequencies of the cavity). This can cause vacuum fluctuations of the electromagnetic field to be converted into real photons so that light is produced in the cavity! Some refer to this as a way of generating light from the vacuum, though it should be kept in mind that mechanical work is required to oscillate the plates, so again there is no free lunch. Nonetheless this effect is very interesting, and was actually observed for the very first time in 2011 (https://www.nature.com/articles/nature10561).