Note the context. Joel is writing his examples in VBScript. VBScript was a dynamically typed offshot of Visual Basic, but did not originally give you the ability to define your own types.

In any modern language you would just use a HtmlString type to wrap html, and let the compiler take care of ensuring you don't mix up encoded and unencoded strings. But absent any kind of custom types (and no HtmlString in the standard library), you had to rely on something like Hungarian notation and very carefully eyeball to code.

Making wrong code look wrong is better than nothing, but making wrong code impossible is far better.