Context is that graph is specifically of people with achievements enabled, which is usually achievement hunters or the types that are invested enough to care about collecting things - so it excludes the types that just do MSQ, roulettes, login casually or otherwise don't care about collecting mounts and achievements due to RP and social activities.
It's still relevant. Its significance is that the achievement/collector types are often the very invested, very knowledgeable, veteran mentor types and it's saying that they are (potentially) giving up despite their sunk cost.
But it's not the same as the graph of players overall which, last I checked, was more around Shadowbringers levels rather than Stormblood levels. Obviously this is because people who have invested a significant amount of hours are more likely to be bored or unhappy than the more casual players that don't register on collecting websites.