I believe they were both "accidentally open" for similar reasons. Neither company produced the most important chips and components in the device itself. That meant that you could assemble a greater understanding of the device than even the manufacturer had and there was good incentive for putting this effort in the early days of computing.

Intentionally open. As noted above, Apple published schematics and ROM source code. IBM published system board schematics as well as the BIOS source code.