The status quo is hard to change. The British are responsible for changing the status quo from “don’t really care” to “illegal.” That also has effects on how people grow up thinking about LGBT rights, which is how you get support for a law that’s ratcheting up the oppression.
I don’t think this article should be read as “the Brits are fully responsible, period,” but as an example of how the many harms of colonialism still leave marks today.