So, as this is a subject that is incredibly dear to my heart, and I'm noticing a bit of anger, I'd like to chime in. I'm a trans woman, and I am very familiar with how most of this works.
After doing a bit of reading, this is absolutely out of context. Yes, anyone that says top surgery for transmen needs to happen before mastectomies for cancer patients is an idiot. That's not what's being said here. This is far more of a criticism of the NHS, which is notoriously horribly when it comes to trans care. For context, it often takes years of being on a waiting list before one can even access trans care including hormones or therapy, and then further years before one is even considered a candidate for surgery. These years are often filled with crippling dysphoria, and to have one's surgery cancelled, if not put off for further years can be a huge issue. Untreated dysphoria can and does lead to fatalities.
The point the poster is trying to make is not "screw cancer patients" but "screw people who don't think trans surgery is life saving and will cut NHS funding until the only mastectomies that can be afforded are for cancer patients".
So, aside from this person not deserving the hate they're receiving, I'd also like to add that basically telling people to "suck it up" is actually really hurtful and marginalizing, because it's what we've been hearing for decades used to deny us access to life saving care. Anywho, just thought I'd offer my perspective.