trans women are absolutely not cis men tho, and hormones develop breast, lactone levels, and make producing milk far more common, on par with cis women
Not 'how tf (the fuck)", an expression of shock, but "How? tf? (transformation)?", a question of the precise mechanism of this transgender breastfeeding scene.
Presumably, they assumed the person in question was a trans woman, (which may not be the case, it may be a trans man, I do not know), and wondered if this was a case of them being physically transformed into a female with functional milk glands, or just handwaved. (Because, while trans women can lactate in real life, it usually requires an experimental drug regimen.)