RMT essentially spreads the wealth much more evenly than would happen naturally. Basically all profits from the mobs that RMT kill are split among all those who buy gil, rather than split among only those who killed. This doesnt cause inflation, but simply speeds it up because when the gil is highly concentrated, inflation over the entire economy will not occur quickly because there is only so much gil those ppl can each spend at a time. It will occur slowly though, unless ppl whore the gil and never spend it.
Anything that spreads gil among everyone quickly, and causes the rich to spend their money, is what causes the quickening of inflation. Selling dynamis currency, selling ancient beastcoins, selling gil online... all speed up inflation tremendously because they provide more ways for the gil to spread from one person to the many suppliers or coins / buyers of gil, causing it to lose value. Without these things, gil would be forced to spread from one person to only a few, and inflation would happen much slower (but it would still happen as long as ppl spread the wealth).
If there is 1 trillion gil on the server, and 90% of it is on 10% of the characters then prices will be one way. When the gil spreads to the other 90% of the ppl, and average gil per person rises, than of course prices will rise. This doesnt actually harm anything except those who keep their assets in gil instead of items, and under the circumstances that the avereage prices break game design (like 9 digits and bazaars).