Not sure what you're questioning, but yes in all my years of dealing with marketing teams between companies, acquisitions and partnerships etc., one of the biggest hurdles is consistently about access to the customer base. Marketers would die for a list of email addresses to market to directly, and likewise the company that cultivated and grew that list is not just going to let it walk away.
By requiring cross play partners to sign up for XBL accounts, MS will have that info. It's a strong move on their part.
Someone asked what about Rocket League etc., my understanding may be wrong but I read elsewhere that they will be handling the servers and whatever registrations will be needed themselves, so it's likely that Sony has already agreed a marketing deal/restriction with them separately, as I assume they have done with any of their other out of PSN partners such as MMO companies.
It very well may be that Sony and MS could not agree on terms for direct access to the customers, and that's the "political" issue that has been discussed. We know that it's not a technical issue.