To be fair, when she was asked abour large outdoor gatherings, she didn't say they weren't an issue, it was more that, in terms of risk rating, they were relatively low risk.
So, there could be transmission at Cheltenham, but, if the people hadn't gone to Cheltenham, unless they'd sat at home on their tod, transmission could, for example, have occurred in pubs and restaurants, and there may well have been more transmission, especially if the races went on behind closed doors.
Politically, to avoid blame, it would have probably been better to cancel Cheltenham, but, scientifically, it didn't make sense to do that in isolation, so you'd have had to shut everything down at the same time.
At this moment in time, with the benefit of hindsight, going into full lockdown in mid March seems sensible, but was the country ( ie the public ) ready for that at the time ?