Unfortunately, in these cases, people make the mistake of thinking the law works like computer code. In reality, it doesn't.

People have this idea that law is just like computer code. You make one single definition and then build laws, like a mathematical edifice, around that definition. They think that if the law uses one definition in one place, it must use that definition in all places. They think the law works like a computer program or a physics equation. Change the constant and changes cascade through.

The law however is not a computer code. It is not a physics equation. The law has not, does not, and will likely never use consistent definitions throughout all contexts. Laws can be written with the same term defined multiple ways in different contexts. A tomato can be a vegetable in some legal contexts and a fruit in others. Someone can be legally male in some contexts but legally female in others.

Traditionally how this works with trans folks is, "your legal sex will be defined as whatever hurts you the most in the moment." Does a trans woman want to use a women's restroom? She will be defined as legally male and thrown out. Does she show her breasts in public as protest? Her chest will be considered legally female breasts. She will then be arrested and thrown in a male prison.

The law is not internally consistent. Don't make the mistake of thinking it is. Usually individual laws have their own definitions written into them. These definitions define what terms mean for the sake of applying that and only that law. And the definitions used can differ between different laws.

I had a look further into this, because I wanted to better understand what factors might cause an act to be considered indecent exposure (or outraging public decency). This led me to some guidance on naturism and other non-sexual nudity, from the crown prosecution service.^1 It appears that having an "intention to cause alarm or distress" may be relevant for protests like this — arguably the entire point of the protest is to use the shock value of the nudity as a protest.

That being said, I think it's a bold move and possibly an effective protest. Even if public indecency laws are gender neutral, it would still be a strong message if any of these women got arrested for this — the reason why these women are capable of causing alarm or distress by going topless is because these are "female presenting nipples" (to use a heavily-memed phrase from the Tumblr porn ban era)

The police in Edinburgh aren't going to do anything. People get their tits 'n' bits out regularly "for art/paganism"

https://beltane.org/about-beltane/