Fair enough about Alembic.
ILM and Pixar are very much Disney employees though, as in employees of the wider Walt Disney Company (which this page is for; the specific page for WDAS is here [1]). If you get a Silver Pass and your pay comes from TWDC's bank account, I'm pretty sure you are a Disney employee. WDAS, Pixar, and ILM are all just individual subsidiaries under the wider TWDC umbrella.
Sony gave it to ILM to start using, and then open-sourced it.
I'd quibble that while ILM and Pixar employees are technically now Disney employees, it's not as if they work for Walt Disney Animation Studios directly, so it's not as clear cut as you make out.