Nonsense. Most of the .tk domains are free URL forwarders; they're not registered to some entity except the registry itself (watch the Whois), and they can't be used except for redireting HTTP requests.
The .tk stats may look impressive but they are only padding out the numbers. Given that most of them are not paid registrations, they really should not be in the table of ccTLDs as anything more than footnote.
Free .TK domains can be resolved to websites, used for email and for redirects, just like most other TLDs. The business model of the .TK registry (freenom) is holding on to expired domains and monetize the traffic with ads.
They are fully functional domain names.
TK is a perfect example of why registration numbers are only a statistic. Just because a TLD like XYZ has a lot of registrations (because of the $1 promo) does not mean end users are going to clamoring to buy the XYZ domains you regged for $1 each for five figures. The more likely scenario is the renewal will go to $10 and most of those cheap registrations will get dropped.
... and you can also RENEW a .TK for free (as long as you do it within the 14 days window prior to expiration). Free renewals are possible for 12 months max, but every year they can be renewed for free for another year.