I can agree ony up to a point because:
By all means, the amount of word you may know is not comparable to the knowledge you've got. Even though you know loads of words but don't use them in everyday speech means you don't know as much as you think you know.
It may be even ample but still you have to take into account how many of them come handy when it comes to speak.
Everything comes down to the one thing. It's not the amount of words you're acquainted with but rather the question; Are they come into their own while nuttering? These alleged words you know may be in no usage so don't talk bunkum that it's possible to estimate how many words you know (including argot thing, proper register or idioms ), you name it. The most appropriate to said would be like; if you are able to use as many words as you think you know in daily talk then you may claim you have a knowledge which enables you to speak easily. A man who doesn't have any problems with English.
What you've said - I mean 'counting words' is good for nothing cause you might whether underestimate or overestimate what you know. There's a possibility of coming across word that at first glance is new for you. However, after a while, you realized it's not. That's why what you can't put a correct estimation on at the numers of words you know. And what seems as easy as pie it's apparently not.
Cheers!!