TBF I think this is just Lullaby Trust - they seem to have gone bonkers OTT again.
This has changed over time - when DS1 was tiny it was FSID (foundation for study of infant deaths) and it was very overcautious with lots of advice about preventing cot death although by today's standards positively laid back XD
Then they went through a more sensible period where they brought out the co-sleeping guidelines - can't remember if this was before or after the Lullaby Trust name change.
Now it seems to have gone more overcautious again.
I know that 6 months room sharing was advised when DS1 was little as had lots of arguments about it on here IIRC. But it was later, around 2013 I would guess, when people came up with this sudden idea that you couldn't put them to bed in the evening and come downstairs, or let them nap in another room. I am not sure where this came from but I know that there was a change in wording in advice and it was to do with the fact that when the advice was about babies sleeping "in your room" people took that very literally and thought it was about the room, not about the parental presence - so they changed it to presence and emphasised that SIDS can happen any time, not just overnight.
I do think though that people tend to interpret guidelines very literally and rigidly today compared to how they were given in the past. I feel like that has made things difficult and more stressful. It is useful information to know that sharing a bedroom has a protective effect against SIDS (even though we don't know exactly why that is) but I feel like in the past people would have gone OK - that's a good, protective thing to do so let's make sure we do that for the longest part of the night but we won't worry too much about an hour here and there if it doesn't happen.
Now it seems like the guideline about what is a protective thing has become some kind of minimum threshold and if you don't meet it then you're somehow neglectful and don't care about your baby's safety.