The fact that Meta removed Threads Software from FB implies that Meta itself considered there to be a likelihood of confusion between the two. They should lose hard here.
And it’s not without precedent to secure an injunction against as US tech company launching a new service - gmail was known as googlemail in the UK and some European countries for several years owing to a trademark dispute (https://www.theguardian.com/media/pda/2010/may/04/digital-me...)
But sadly a lot of trademark and copyright cases come down to who has the bigger pile of cash behind them.