Sorry for the late reply, I've been considering this and I have come up with two answers.

The first is that befriending an out-group relays some political advantage as a long term stategy. This was certainly the strategy undertaken by Atahualpa with the Spanish Conquistadors. Arguably a tactic taken by many groups when faced with a technologically advanced intruder. Menelik II played this strategy very well at the end of the 1800s, welcoming the Italians, acquiring all the technology and then expelling them. You might've missed my article about him - as I think it's before your time. https://medium.com/p/4e3f85a9a31b

I think history is full of that sort of interaction, the outgroup is allowed access to the in-group but what appears as benevolence is really a holding pattern pending more information gathering. Keep your friends close but enemies closer being the maxim of choice.

But that isn't the full picture... I think in any tribe where there is an abundance of resources (and a more egalitarian system of exchange than capitalism), any non-threatening member of an out-group would receive a warm welcome. Capitalism naturally creates winners/losers and a hierarchy but a financial one doesn't necessarily exist in all cultures. Capitalism (and the industrial revolution) have super charged the resource hoarding of nations.

Prior to this, in Western feudal systems and in tribes/smaller groups around the world - what was really being traded was reputation. In a more financially egalitarian society, your reputation is what facilitates trade, it's what brings in trading partners and ultimately what acts as proof of whether you are worth bothering about. If you cannot wage war and take what you want, then the only other strategy available is to shore up your reputation and buy/work for it. Faced with someone from the outside, the inclination towards benevolence is a good strategy. This would be particularly true in areas where there are multiple tribes and little homogeny. Unless you are the biggest kid on the block - you need to amass goodwill. We see this in Chimpanzee troupes and other primates.

I don't think hospitality reduces the in-group/out-group thinking necessarily, though I am prepared to concede over a period of a hundred years or so, it my do exactly that. I still think it is there and I still think there would be issues integrating two groups without introducing a commonality of perspective. One could make the argument that the concept of 'The United States of America' does exactly that, though not entirely successfully.

This was a fun little thought experiment and I'm glad you raised the question. I feel like I've written an essay in response, but I'm interested to know what you think. :o)