Don't agree with that at all that is what the article says. (It doesn't match up with my definition either based on my years of business experience either.)
Definition of "speculation" from investopedia:
"The act of trading in an asset, or conducting a financial transaction, that has a significant risk of losing most or all of the initial outlay, in expectation of a substantial gain."
Based on history there is no reason to believe that the purchase of a taxi medallion could result reasonably in losing "most or all" of the initial outlay. (Unless of course you decided to buy one while ignoring the current events going on of course).
http://www.investopedia.com/terms/s/speculation.asp
Of course you can make any investment or purchase speculative depending on how much of a chance you want to take.
If you decide to get in on an up and coming area in real estate (a run down area that you hope will change over the course of many years) that could easily be seen as speculation. Otoh, buying a condo in a well established neighborhood that has a history would not be seen as speculation which is not the same as saying that you bought because you felt that the value could double and if that doesn't happen you may feel you have "lost" something (the high gain you expected).
Even in the case of eminent domain taking of property for public good the property owners are compensated somewhat fairly for the loss of value of their property. The government doesn't come along and say "hey we need this area for a road sorry guys to bad it happened to you".
I would go further to say that these medallion owners would band together and at least attempt to do whatever they could legally to find a way to prevent something like this from happening if they could. Why? There is enough money involved to pay lawyers to muck up the process or delay it (just like with eminent domain, people even if there is compensation, will do that.)