Their source is StatCounter. I would trust the trends but not their absolute values. This month, Steam reported an increase to 4.01% while Cloudflare's human activity reports ~6%.
Even the trends should be taken with caution. By the very nature of their collection methodology and Firefox’s user base, they massively undercount Firefox (almost certainly by more than 30%, from the last estimates I remember; and possibly significantly more). I would expect them to heavily undercount Linux for similar reasons.