The checked version of their app is v3.9.8 which seems a few versions behind the current branch but still fairly recent.
The blogger discovered that a user's Google mail address along with the advertising ID and a bit of other info are sent to Iterable, AppsFlyer and Tune along with some Google services like Analytics - all seemingly without the user's consent and even without mentioning it in the app's ToS.
Of course customer support has been asked as well. Their answer was not satisfactory:
Hello there! We use these tools to monitor aggregated data to improve UI/UX and determine the efficiency of our marketing campaigns. They are not related to the user’s activity when using our VPN service. In case you have further questions, do not hesitate to drop us a DM!
Everyone interested in some of the HTTP POSTs discovered can look at them in the article linked above. The article itself is German-language, but it doesn't contain more info than this, only a bit of the writer's opinion which I share: It's very questionable that a "no-log" or even "privacy-centered" VPN provider like NordVPN is bold enough to state "marketing reasons" as their justification to track users of their Android app. Even worse that this tracking is performed by third parties who will most likely use this data in cross-referencing...
Try to avoid NordVPN. Searching for "NordVPN" in this forum alone will yield more than enough reason. One in three newly created threads is about them.