Why are some converters more complicated than this? Because you can make them a little faster. On my laptop, glibc's sprintf(buf, "%.50e", M_PI) is about 15 times faster than an equivalent print using the code above, because the implementation of sprintf uses much more sophisticated mathematics to speed the conversion.
(When did he start writing articles in Go? IMHO the syntax is close enough to C to make skimming through easy enough, but confusing to look at in detail for the majority who will have likely far more experience with C than Go.)
He missed an opportunity to name the article "Floating Point to Decimal Conversion is Easy and Not Fast" (in reference to https://swtch.com/~rsc/regexp/regexp1.html)
(When did he start writing articles in Go? IMHO the syntax is close enough to C to make skimming through easy enough, but confusing to look at in detail for the majority who will have likely far more experience with C than Go.)