Ryzen 4000 series (which is for laptops) is relatively easy to get, but I don't fault the author for not buying a laptop specifically for this benchmark. The 3900x still pulls more power and is on the same technology as the 4900HS/4900H so it's definitely not a bad-faith comparison.
Not exactly. The 4000 series part have a finer lithography for I/O and use a low power process.