I’m not sure why you are asking the question when the answer is obvious.
Any company (and in particular a start up) is going to rely on third parties. How often do you read on the news that Lloyds banking goes down? FT aren’t going to have 100% uptime contracts with every third party - do you realise how much that would cost? If you ‘need’ that then you are probably going to have to pay more than £0 - £10 a month.
FT aren’t going to personally restrict people buying shares - they’ve made that clear. But if a third party shafts them without warning it’s unfortunate and out of their control - and could temporarily stop orders being placed.
And it’s also important to point out at no point could you not sell GME - they were only forced to limit buys.