I've just checked a largish repo, and the node_modules folder is just under 500mb - I could check that in, and be done with it. Updates aren't constant, so it would be every now and then.
That's really not a lot for knowing the code that is being used in your codebase hasn't changed, and the bonus of having everything available should the registry go down, or something.
So I don't think it's 'horrible advice' - it's do what suits your needs best. Some people want to have everything they need to build their application in their control, on the off chance everything hits the fan.
(Also, this: "Don't break the industry practice and check-in your node_modules" - does not necessarily mean it is the best way, it just happens to be the advice from the start.
That's really not a lot for knowing the code that is being used in your codebase hasn't changed, and the bonus of having everything available should the registry go down, or something.
So I don't think it's 'horrible advice' - it's do what suits your needs best. Some people want to have everything they need to build their application in their control, on the off chance everything hits the fan.
(Also, this: "Don't break the industry practice and check-in your node_modules" - does not necessarily mean it is the best way, it just happens to be the advice from the start.