These are not just web developers though, they're full stack developers; perhaps pure frontend developers are more prone to use Apple products?
At least far more than Apple's general market share.
Even most designers I know personally don't use macOS. But if I was going by what I see at JS conferences, you'd think Apple has a 99% marketshare (although things have improved in recent years with "full stack" bringing in Linux folks and Microsoft's investment in dev tools making Windows more appealing).
This is similar to how everyone at these conferences seems to have one of the latest iPhones yet (depending on where you live) the majority of smartphone users uses Androids and those who are using iPhones generally hold onto them for years before upgrading.
Also FWIW as a long time JS dev, it seemed to me like Rails actually was one of the major contributors to the macbook monoculture because Rails (in the early days) was popular with startups who also often focused on iOS apps first, which means they had no other choice than to use macOS for development. As iOS became less of a focus than web dev, the macbooks stayed and only now seem to be slowly replaced as Microsoft is making a heavy push into web dev relations and the opinion of Apple's quality gets tarnished by dev-unfriendly decisions and hardware problems.
Yes, but they're also more influential in the respective communities, and more probable as presenters to be writing stuff others use (e.g. this Firefox dev tool).
So they'll use what they personally use to develop.
>Also FWIW as a long time JS dev, it seemed to me like Rails actually was one of the major contributors to the macbook monoculture because Rails (in the early days) was popular with startups who also often focused on iOS apps first, which means they had no other choice than to use macOS for development.
I don't think that's the case - Rails confs/devs was mostly Macs before iOS apps where a thing (which happened 4-5 years after RoR was released).
But this may be a regional thing (so selection bias again). When I was freelancing as a Python web dev, the few places that used Python usually used Plone/Zope (barely anyone used Django or Flask) and there were even fewer places using Ruby except startups. The popularity of Plone/Zope seems to be very much a German thing, just like SuSE Linux and Typo3/Drupal (instead of Wordpress). We've even had a very large Firefox marketshare until fairly recently (because of privacy concerns over Google Chrome).
That said, in European JS conferences over the past five years I'm definitely seeing more and more non-macbooks each year.