Re: How?
Not one of the downvoters you refer to, but I think the arguments are:
The site DOES have more than one point of connection to the grid, but there is a major (grid) bottleneck at North Hyde - this is the DNO (SSEN)'s problem and NGET's problem.
Heathrow itself -should- have had sufficient backup power, but apparently did not. (the grid should be expected to go off sometimes.. it last did so in 2019 iirc)
And even if Heathrow did have more backup power, the blackout was quite widespread, and while they might have been able to land a few planes, they might still have had to divert others due to wider chaos..
It's not so much a Heathrow problem really so much as a UK problem IMHO - decades of privatised utilities have meant a chronic lack of investment, and with the London grid load going through the roof due to datacentres, EVs and the like, the old 1950s supergrid transformers (the one that failed this morning might be 70 years old) can no longer cope, and need replacing right at a time when HV grid transformers worldwide are many times more expensive, and harder to get hold of than ever before, with lead-times stretching to 3 years