They are not backtracking, HSBC have never said they would move their HQ out of London. Since 2/3 of their business is in Asia it doesn't make as much difference if London is in the EU or not compared to many other banks.

What HSBC have said is that they may move about 1,000 jobs to Paris. That's because those jobs are tied to services that by law must be provided within the EU legal and regulatory environment. When London leaves that environment those jobs will go, not just at HSBC but across our financial sector. See my other larger post on this page for further details.