There is insufficient evidence as to whether myopic slaves were more valuable in Ancient Greece. The passage cited is by a ophthalmologist (Kaisu Viikari), not a historian, and he has assumed too much from the evidence we have.
The price of slaves (see here for some examples), as with most 'commodities', depended partly on supply and demand. While we have a fair amount of evidence on this for slaves in general - and on some details such as age, origin, sex, education, attitude and physical condition - there is no information on slaves with myopia.
How the Ancient Greeks (and others) managed to achieve such fine details on, for example, coins, has been the subject of debate among scholars for a long time. Unfortunately, we have no firm evidence one way or the other as there is no mention of this in any surviving sources, so we can't really be sure of the value of myopic slaves. A myopic slave, though, would not have been of much use to most slave owners so the value of such a slave would probably have depended on the owner: if he wasn't in the business of making coins or crafting fine objects, such a slave would have had little value. In fact, there was a law in Athens about selling 'diseased' slaves - the buyer could get his money back.
Of possible relevance to usefulness of a myopic slave, this Penn Museum article (which cites several academics) says:
To many, a puzzling mystery of the ancient world is how minute
artifacts or parts of them were made without magnification. Dramatic
examples of this include small engraved Greek and Roman gems...coins with tiny engraved letters (Fig. 2a, b)..... and the intricate jewelry of the Greek and Islamic Periods.
The article goes on to hypothesize that, firstly,
Close work was done by craftsmen who, due to their myopia, had
excellent close vision and the capacity to see objects magnified.
An increase in the number of myopic craftsmen took place in fields
requiring close work due to (a) the genetic nature of myopia, (b) the
cultural patterns of the society, its ideological strictures and
socioeconomic organization which led to the relaxation of selection,
i.e. as opposed to natural selection.
In support of these hypotheses, it is argued that hereditary myopia fitted in well with the tradition of fathers passing their craft on to their sons.
Social immobility, consanguineous intermarriage, the genetic nature
of myopia, plus the familial craft pattern would lead to an increase
in the proportion of the dominant myopic trait
It is also possible some of these craftsmen were originally slaves who were manumitted. One could argue that the value of a myopic slave would quite likely have been recognized, but there does not seem to be any firm evidence for this.
On the other hand, there is evidence that magnification was known and could thus have been used by craftsmen or slaves owned by them. How limited or extensive the use of lenses was would be relevant to the value of a myopic slave.
In The Use of Magnifying Lenses in the Classical World, Harvey Hanna notes the controversy surrounding finds of magnifying lenses. There is literary evidence from both Pliny and Aristophanes as to the use of glass to make fire, and Pliny also mentions magnification, but not in the context of artisans. Nonetheless, Hanna (referring to lenses) notes:
that example have been found in the workshops of artisans. This would
indicate that at least some artists were using them in their work. Why
have so few been found in context? The answer could be that these
lenses were very valuable objects and were well taken care of, being
passed down from father to son as heirlooms.
None of this, though, would mean that myopic slaves weren't used and even valued. Given more modern evidence on myopia and education level, it is certainly possible that some myopic slaves were well-educated and thus worth more. On the other hand, the myopia could have developed through intensive close work after the slave was acquired. Aristotle noted the distinction between far-sighted and near-sighted but, with an estimated 90% of original scientific sources lost, we don't have much more than this.
Edward M. Harris, Democracy and the Rule of law in Classical Athens
Matthew Dillon and Lynda Garland, Ancient Greece