If it were purely self-serving, they’d agree to settle with Google on special terms similar to those offered to a number of other publishers. Court proceedings last for years, as would be obvious to Sweeney, so sacrificing years of revenue for the distant prospect of having to pay 0 to the platforms is easier explained as being ideological than a business decision.
The store came to be as an “f you” to Apple, as evident from the email exchanges at the time. The 1984 trailer from Fortnite seemed to be in the same vein. I don’t think that particular trailer was for the purpose of promoting the EGS, as there’s not a single hint to the store in the video or its standard placement.