That... Has nothing to do with what's going on here.
They claimed their "wifi" was the "fastest" because had the highest "minimum guaranteed speed". That speed is no where near 1Gbps. It guarantees 30Mbps.
This is not about having a 1-5gbps backbone, it's about convincing people that basically the same wifi is faster than the competition, which is extremely misleading.
Even if their backbone is faster, that's not even the argument they're making. And even if it was, it's not like people are likely to even notice that difference.
I don't think "anything close" is even vaguely true. I have openreach FTTP at 900mbps down, and the bandwidth is the same or better than I ever got with virgin gigabit. I'm also about to switch to another FTTP provider who provides 900mbps down 900mbps up for £25 a month. Plus with both of those I can pay a little extra to have a monthly rolling contract.
Additionally my average latency dropped by more than half when ditching Virgin. I was genuinely shocked at how much better it was.
The other provider is Toob and they are indeed quite limited in location currently. I still pay less for a rolling contract with Cuckoo for my openreach connection that I did for Virgin gigabit (by ~£10).