I suppose it's possible to have a difference of opinion on the meaning of "open". The BIOS was the only bit of the design that IBM had any IP in, and they definitely intended other hardware manufacturers to make compatible products, if not perhaps entire systems.

It's important to remember the regulatory environment that IBM had operated in at the time.

For more than a decade it had been illegal for IBM to exclude 3rd party hardware makers from its mainframe market, beginning with the so-called "plug compatible" peripherals, which resulted in publishing standards for the IBM Channel interface, the prohibition on refusing to allow e.g. 3rd party memory for mainframes, and importantly, the requirement that IBM license its operating systems and software to run on any manufacturer's hardware.

This was the environment that the PC was designed in. The (IMO) open design - which was not an accident - was really the only possibility that IBM had while still holding to those earlier legal restrictions on bundling, tying, and license restriction.

The fact that Compaq could reverse-engineer the BIOS was inevitable given the market opportunity. IBM tried to hold on to that thin reed but it was a doomed effort. As the "PC Compatible" market took off IBM became irrelevant to much of the growth in the industry.