Most of their "long term securities" are actually short term by any reasonable meaning of the word. It's just a technical terminology thing for filings that doesn't mean what you assume it means. They have been north of 40 billion for a while.

The relevant thing is how much is liquid fast enough to use it for what they want to use it for. For that we should listen to Apple telling us directly (which they have repeatedly done), not just check some field on their govt filings.