Is it true that you cannot mix any antifreeze in a 2000 VW Beetle GLX 1.8 turbo except the VW antifreeze G12A8D that is only available from the dealer for 25 dollars?

Yes, you must use G12 type antifreeze which is similar to Dex-cool but they are not the same. But that doesn't mean you have to buy it from the dealer at $25. There may be other sources. PEAK® Global LifeTime™ claims to be a replacement for G12 but considering the importance of using the proper coolant, I would use only a G12 coolant.