It's obviously not a gimmick. It ties the date in UK law regardless whether the EU want to vote for an extension. Deadlines are good for focusing the mind. Otherwise some will and procrastinate thinking they can delay things forever. With this in UK law the EU (and the UK parliament for that matter) will know they can't mess about, add an extension, then mess about, add an extension, then mess about, add an extension, then mess about, add an extension, then ...
Perhaps this passing into law will wake up industry/commerce in the EU, and get serious pressure put on the EU "negotiators"/delayers to talk about trade and come to an agreement.