Not sure, I didn't try, but using one was a good excuse to learn how. The blog post suggests that despite there being 8 they're not all suitable for use (maybe due to high on boot or something)
Ah... that makes sense. It looks like IO0 needs to be pulled high to boot.
Still looks like you could redefine RX to a GPIO and still have 8 if you didn't need serial input. I vaguely remember running into this issue with some load cell AMPs.
A Shift Register or IO Expander seem like the more interesting and less frustrating route. I'd probably end up using an ESP01 and i2c IO Expander because I'm a masochist and love trying to squeeze as much as I can out of the ESP01.