The Tofugu article says that when used with a noun, かな follows the noun without any additional particles, e.g.:
それは人間かな
However, they later give an example where なの is inserted in between a noun and かな:
アメリカは本当に自由の国なのかな
Why is なの used in one example but not in the other?
(Tofugu puts the second example under the "な adjective" section; but I guess it's a typo, as かな follows the noun 国, not the な adjective 自由.)
As you've already guessed, the explanation for the example is wrong, as 国 is not a な adjective. It is still a valid sentence, the なの is the same as the last の in "これは、会社の建物だったのかな。" which was briefly explained on the Tofugo article. It means sth like "The thing is" or "is it?" and thus goes well with かな. Also the second sentence puts more emphasis on whether it is really the case (that america is a free country), which is expressed through なの. You could translate the sentences as: I wonder if that's a human. I wonder if it is really the case that america is a free country. なの would be "the case".
