Both cross-play and cross-progression were included as part of Rocket League’s free-to-play launch last September, but it appears that there were plans to test a mobile version Rocket League Items of the main game before releasing any of those features.
Because the documents are from last June and the slides with these details were listed under “old,” a lot probably changed before Epic and Psyonix announced the game was going free-to-play a month later on July 21. Whether that means a potential mobile launch for the main client of Rocket League is still happening or not is currently unknown.
Epic did note that, as of June 2020, Rocket League was recording more than 10 million active users across all www.lolga.com platforms, making it the highest since the game’s launch in 2015.