Taco Bell is the one who identified one of their lettuce suppliers as a source of infection. (There are likely other sources, as some of the infected do not eat at Taco Bell.) Taco Bell then announced the issue (but did not name the supplier), pulled all lettuce from that supplier from stores and warehouses, plus anything else they thought might be questionable. They seem to be being very responsible here. (Admittedly, there was a long initial delay, but they were probably expecting government agencies to still be functional and doing their jobs.)
In many scientific tests, you need to chose between false positives and false negatives. The way to run infection tests like this is to choose for false positives: if a sample is negative, you can be sure that it's negative and you can ignore it. If it tests positive, you can re-run the tests with other criteria and confirm or deny the original results.
It seems like the regime, tired of being blamed for not figuring this out, got a false positive and made the announcement before re-testing was complete. And now Taylor Farms (which has been involved in multiple other past infections) is saying that it wasn't them, almost certainly because they've lost business, and the FDA is being forced to back them up on that.
I'll also add that it's been, what? two months since this started? It's possible that Taylor Farms cracked down on their produce and farming practices, and that the samples provided to FDA are genuinely clean and we may never get a definitive answer; that's happened before in other outbreaks. It's extremely unusual for there to be no definitive answer in an outbreak that's lasted this long - but then again, we used to have a functioning government.