IG dont own GME stock even. The business of spreadbetting is banned in USA hence options and so on are mentioned on reddit I guess. Indirectly IG or whichever company will cover their exposure by holding options but its also possible they just ride the risk in cash and rely on the customer selling a loss as it swings against them. UK taxes these companies very usefully, we have some of the best laws in this respect. Its a bit like insurance is how I think of it, events that may or may not ever occur.
GME is useful in that we'll see spikes again elsewhere, like on a wider basis I hope in silver or gold but I dont know where exactly I just presume it will not be so specific
https://en.wikipedia.org/wiki/Silver_Thursday
Google or watch silver spike 1980 on youtube I reckon is relevant