Making it airtight would actually increase the risk of botulism, because the bacteria is anaerobic.
Also, the botulism bacteria wouldn't survive such an acidic and salty fermentation environment anyway. Have a better chance of getting both struck by lightning and winning the lottery multiple times in a row than dying from botulism, even in an environment that's ideal for it.
I wonder why botulism even gets a mention in modern discussions of traditional fermenting and food preservation.
It's unbelievably rare and really only occurs in modern preservation methods, like canning, and commercially-prepared food.
I just don't understand why it gets a mention at all. When I see it pop up in tutorials about traditional curing and fermenting, it's an immediate red flag about the quality of the tutorial. It's one of those common misinformation things that won't die because of all the momentum behind it.
And the misinformation ought to stop, because it's giving us cancer, because industry and home-curers think they need to load nitrites into their meat to make it safe, which is terrible for you.