Please accept my apologies (or my apology, as the case may be :-)) for arriving so late: I've only just bumped into this grammatical situation, I did a search and none of the answers seen here really works for me.
My view is that, no matter how "jarring" the mixing of numbers may sound, in this case - or, should I say this kind of sentences? - what dictates the grammatical structure should be the meaning of the noun used as the object.
So, "kind" is a set and, of course, a set is made of many similar things: by definition, it cannot possibly be made of only one thing, no matter how similar the units are to one another. As often encountered in mathematical discussions, one speaks of a "set of numbers" and not of a "set of number"; a "set of functions" and not of a "set of function" and so on. You may give this set a name such as, animal, plant, mineral. Then, if using the name, it would make sense referring to it in the singular (e.g. what kind of animal is it?). Therefore, if the author is talking about just a type of objections received from Forbes, then he is perfectly entitled to write "this kind of things" whereas, more than one type (i.e. a grammatical objection, an objection about design and an objection about content) would suggest the use of the form "these kinds of things".
For those who advocate usage as the guiding principle, then Prof. Google tells us that, on this date, people use this expression as follows:
- this kind of thing: 136,000,000 hits
- these kinds of things: 24,900,000
- this kind of things: 10,900,000
- these kinds of thing: 1,960,000
So, the most used, or most popular, is not necessarily the most logical, but wins the day. Which just goes to show that the majority of people are irrational! :-)