Wow, neither the article nor any of the comments so far have worked out that this is not due to the updates being mandatory...
(Seriously, a whopping 7 updates have been released so far
and I can't imagine any of them being multi-gigabyte downloads yet)
No, the culprit is rather Windows Update's new peer-to-peer implementation which shares the updates you've downloaded with other Windows 10 users over the internet.
Just like turning one's PC into an always-on bittorrent seedbox, this can chew through one's bandwidth cap like no one's business, especially if one's ISP differentiates between upload and download bandwidth caps.
Fortunately, as my screenshot shows, you can disable this, or enable it only for other PCs on your network. Not only does the latter option not increase your bandwidth usage, if you have multiple Windows 10 devices it can actually
decrease it by letting the update download only once and be shared among those devices.