"Friend told me that once device is locked via iCloud, not even Apple can find out its location. Is that true?"
I have no idea if it's true or not; you'll need to ask Apple and we are not Apple.
I'm sure there are instances of FindMy not working, but at this point does it really matter? The Mac is gone and you say it's locked. If it's useless to a thief, he/she will likely just trash it. Or sell it to some unsuspecting party which puts you right back where you are now.
From the horses mouth:If your Mac is lost or stolen - Apple Support. Which you're already read per the referring article.
Note the section on FindMY not being activated which is the as it failing.
There comes a time when you've done all you can do and you just have to live with the situation. After seven months, I feel that time has come. But that's your call. This forum has addressed your issue in the only way it can. We are not Apple, and we cannot advise you any more than has already been done.