As far as I know, except for Galatia, Turkey, the Celts did not really settle in great numbers in the Balkans. They did stay in Greece and Bulgaria from 280 to around 230 BC, but most of them left. Only those in Galatia settled down and mixed with local populations. Bulgarians are not of Celtic origin, or we Greeks are as well! There is some Celtic influence and racial mixing of course, but not to a high degree....
There was some Celtic presence in the Balkans for decades (in some areas like Greece) or centuries (in some regions in Turkey). I am not sure for how long the Celts remained in Bulgaria/Thrace, but I don't think it was for centuries.
There was during the 3rd century a Celtic empire of Galatoi at the Tundza river and the Maritza, known as empire of Tylis, founded by Kambaules. I suppose the article is about them.
I can't say if bulgarian politics denied this or not. But their role is minor than those of Thracians. But Bulgarians are neither Celts nor Thracians.
As far as i understood from the site, it doesn't claim that Bulgarians are of thracian or celtic origin, but that some archeological artefacts are falsly (but deliberately) presented as thracian, even though they are supposed to be celtic..
But then Anna James rised a good point in the other thread.