I dont think that achievments tell anything about a gamer. You can be a absolutly hardcore gamer without any archievments, i found it a nice when microsoft released it for their x360 in 2005, but i never played a game to get achievments.In my opinion you are a hardcore gamer if you try to play a game perfect and not to collect all achievments.
Example: if you played BF1 since release 1k+ hours you are a hardcore gamer, but if you dont play the story you dont will get 50% of the achievments and if you played it 1k+ hours since release you dont will play many other games and dont will collect alot of achievments, in your opinion theese people are casual but they not!
There are exceptions to the rule of course. Call of Duty where many people do not play single player is a good example. When it comes to games like that though, they usually have a reputation that goes with it. If that is all they play, then they fall under Masters of Their Trade.
Most games however have trophies and achievements that do pretty well at tracking how you play the game. Like what side content do you complete, did you complete the main story, do you play stealth or guns blazing, there is pretty much something for everything.
Also one thing to keep in mind is that I am not talking about judging off of one or two games. The average of several games and the content they have completed does a very good job telling how someone would play. So if they play ten games for example, but because they skipped the story in CoD, the data gathered from the other nine games does not become invalid.
Another reason you have to look at several games is because what if they did not like a game and quit early? You need to look at their overall habits between several games to get an accurate picture. So the argument of looking at one or two games falls flat, as that is not what this topic is about.