It's fair in the sense of comparing equivalent situations. When you're making that claim that Treatment A is available to X, but not to Y, particularly if you're claiming this is targetted discrimination against Y, then it's important that "Treatment A" is actually the same thing right? If what you actually mean is that Treatment A is available to X, while Treatment B is not available to Y, where A and B are actually different things, happening for different reasons, with different criteria, then that isn't a "fair" comparison.
Precocious puberty is a physical condition with observable physical symptoms. It may cause distress. Many/most illnesses or conditions cause distress, that's why medicine exists and why we treat things, this isn't really that meaningful. However you want to frame it, it's a different thing.
Are there examples of cis children who are beginning, or about to begin, puberty at a normal time, who are treated with puberty blockers simply because they are distressed at the thought of going through puberty and want to delay/avoid it? I'm not aware of this happening, but if it is (and in particular is happening on the NHS) then this would actually be a "fair" comparison to make.