You can't appeal the denial of a temporary injunction. The injunction has no impact on the strength of the FTC's arguments at the Administrative Court trial to block the merger pemanently.
MS isn't confident at all, they just want to have the Hail Mary of the early closure in spite of the CMA should the temporary injunction not be granted. The FTC would 100% appeal if MS loses the Administrative Court case and appeals it to the Federal Court and manages to win. The only way they could have solid hopes for a win on federal appeal would be if they'd gone ahead and closed the merger.
Only issue there, and one that nobody seems to have caught on to, is that there is third party involved in the merger agreement. The Securities and Exchange Commission of the United States government. The merger agreement lays out the terms between the first and second parties involved, and pledges to the third party, the SEC, and if they do not hold to the terms of the deal, the SEC can take action against both parties, including but not limited to asset seizure, bans on sale or trading of their stocks on American stock exchanges, forced supervision of the boards of directors, fines and even criminal charges based on the specific acts involved. The SEC is generally seen in American business as the one government authority you NEVER want poking around your company, as their processes can cripple a totally innocent business for months or years, and they can really sink their teeth into a company that has broken rules or laws. They also can sic the FBI, US Marshalls, Secret Service and IRS on a company and its executives if they see reason to, as they are under the Justice Department and are a law enforcement agency in their own right.