The Murcielago, Mclaren, and GT3 are all now 10+ years old. Tire technology has advanced very rapidly since then. Circa 2000, a car with .90G lateral acceleration was considered exceptional. Now cars frequently approach 1.10g with some of the more exotic tires. That is an immense difference.
Sure, cars like the Murcielago and the SLR are not reliable competitors for the Giulia, they put them there just to create that surprised "ooohh" from the not-so-informed viewer. Other than the things you said, there's the "fear" factor: they're proper exotic cars, hugely expensive, and usually they were tested by magazines. Of course the driver has a different limit in mind, especially beacuse we're talking about the Nurburgring. For a car like a Giulia, with a lap-time organized by the company itself, they can afford to even crash them just for the sake of marketing.
It's strange to see the Giulia being faster than a GT3 (2009, not exactly a century ago in terms of tires) and the new M4. The Porsche is born to lap that track, it's way lighter and track focused than a family sports saloon/sedan, with more downforce too. The M4 is the benchmark in that category in terms of handling and on track performance, and it's 13 seconds (!) slower than the Giulia.
The Giulia is more powerful and maybe even lighter than the BMW, but especially at the 'Ring it's not only about power to weight ratio, it would be a huge step forward in terms of handling and setup from recent years Alfas (apart from the 4C).
I think that lap time is very suspicious, more than other 'Ring laptimes, which are always questionable. Let's wait a fair comparison from a reliable magazine against its competitors and we'll see if it's really that fast. If that will be the case, huge credit to Alfa engineers.