But that isn't following the códigos



(I'm shocked! Truly shocked!).
Not sure which particular
To me codigas are no more than mannered or well behavior codified. That doesn’t mean it is inflexible.
My acquaintances who frequent BsAs for prolonged periods, inform me that, in order to get dances there, you must go separately, even if known.
Never been to BsAs. All my knowledge is second hand either through blogs or stories from friends who frequent there.
My impression is there is no homogenous milonga culture. It varies depending on which milongas you go to.
I too have read that if you enter together then it is assumed you are couple at least for that evening in the dance context. But you can still make present available to dance with others.
With so many milongas, I doubt it is followed universally. Though anywhere else in the world too, if you see clearly that two people have arrived together, that they are either friends, acquaintances, or actual couple. Then you observe if they prefer to dance with each other or with others too. I don’t think that observation happens due to conscious efforts.
Also, similarly from some reports from the US.
Reports about what exactly?
From personal experience there may also a more dated approach in parts of Austria (the male will be asked for permission...), but you have to observe the game in play at an unknown milonga.
Asking for permission vs being courteous to acknowledgement presence is a debate that will never end. Not only in tango but other dances too. Many or some in the west take offense to it for reasons well known, while others don’t find it offensive for reasons well known. That debate will never end. Women don’t have to do asking of men. Therefore women have far less chance to ask permission or acknowledge man’s partner when wanting to dance with the men. But I seen it play out subtly. The man asks a woman. Woman accepts but also looks at man’s partner. It also depends of degree of familiarity between who is asking and who is being asked. If a couple is seating together formally and you want to ask a woman, you might nod to the man or acknowledge his presence. I don’t find anything wrong with that. On the other hand if it is more casual, and a couples together have been seen clearly dancing with others or you kind of known them both or one of them, you could ask the lady without giving a glance to her partner. That works too. It really depends on the situational context.
Local cultural rules have subtle quirks and not all countries are as 'liberated' (ah, 'freedom'

) as some like to think.
I don’t think “liberation” or “freedom” means lack of etiquettes. I have said this many times. In a bar or anywhere else if two men are talking and a third man walks into join them - he will acknowledge both. Or people want to steal one of the two people in a conversation, they will seek permission of the other person. Nothing to do with the gender.
Does that means there is no machismo in certain cultures or countries or in dancing? Not true. You could have men who will feel their pride hurt if you ask a lady with him without giving him a smile or nod (which locally may be interpreted as seeking permission).
I have also found very liberated feminist women that can feel it a bit offensive or rude if their partner is totally ignored or disregarded when seeking a dance.
It is all about EQ and etiquettes no matter whether liberated or conservative settings. A good EQ takes you a long way.