These gases were used because they are rare. To be of any value, the gas has to be inert, so it doesn't undergo a natural fractionation (all biological systems favour lighter isotopes). We also need a very old reliably dated source. These gases are used as an indicator for the atmosphere as a whole, and provide the best proxy. Isotope data can be difficult to interpret at the best of times, it's best to take these hypothesis with a grain of salt.

At my university we had a small orientation before we were let lose supervising undergraduate students in labs. One of the topics discussed was the difficulty in teaching someone with preconceived ideas, and how many people have the wrong idea on very 'fundamental' issues. We did a simple test, get into groups of three or four, and produce a little presentation about why we have seasons and why the moon has different phases. This seems like a simple ask, also keep in mind that we are all either final year or postgraduate science (physics and chemistry) majors. Most groups got it wrong. Some fundamentally (the earth creates a shadow on the moon) but most had the basic ideas and failed in the reasoning (such as the tilted axis causing seasons, but fail to realise it is due to flux and related it instead to distance). It's easy to scoff at people who got this wrong, but this wasn't Joe public; this was a large group of scientists, each beginning to specialise in a very technical field. The teaching of basic scientific principals has been left for dead, and the current climate of students is that if something is not assessed, it is not important. Gone are the days that students would look for these answers on their own. Something to ponder, could you imagine the headlines if a reporter was in the room?