So "transition treatments" have gone up 4000% ... in the time period following the treatment becoming available. If being a gymnast was illegal until 2009, or nobody had invented a trampoline until then, you can certainly bet making it legal or possible to do floor routines would result in a 4000%+ increase in people who were openly and publically gymnasts.
Trans people, trans kids, have always existed - we just didn't have the technology to provide the treatment in that article.
That article is choosing to cite the numbers on the treatment rather than the condition because the treatment's very recent launch means it allows the presentation of a scarier number.