I think that 30,000 is an exaggeration, and if so, I just don't like when people sensationalize figures and bend facts to their liking.
The problem would lie in this report [0], which is cited by the article and by the person tweeting about it.
Happy to be proven wrong; but I know the area quite well (I grew up in Italy), and I immediately thought that the satellite picture of Siena depicted in the article, and in the tweet, does not contain 30,000 people.
The entire "Comune" of Siena (roughly equivalent to a "County" in the UK or US) is ~53,000 people [1].
However, I would be surprised if the part of Siena depicted in the article, the city center, houses 30,000 people.
See for yourself. Here's Assisi, roughly with a similar density as Siena, which has ~4,500 people living in the city center [2].
Here's Siena [3]. Just by approximation, I'd say that Siena's city center is in the range of maybe 15k? Not more than that.
Also, it would be unfair to consider the fact that Siena's city center is also house for students, which live in different conditions than normal people. In that sense, even Harvard's dormitories have a high density, compared to any road structure in Texas.
Anyway, just trying to make you reflect on the numbers - I hope my calculations are correct; or at least, the reasoning behind it should be good enough. As said, happy to be proven wrong.
[0]: https://assets.publishing.service.gov.uk/government/uploads/...
[1]: https://www.comune.siena.it/Il-Comune/Servizi/Statistica/Pop...
[2]: https://www.google.com/maps/place/53100+Siena,+Province+of+S...
[3]: https://www.google.com/maps/place/53100+Siena,+Province+of+S...