I'm not sure if the distinction between free and open source software makes a difference for this question, so I'll use the term FOSS.
Can a FOSS platform make API calls to proprietary systems, such as Wolfram Alpha?
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Yes, that is possible.
Even for a strong copyleft license like the GPL, when two applications communicate "at arms length" with each other, then the two applications are considered to be independent works and their copyright licenses don't affect each other.
The determination if two pieces of code communicate "at arms length" revolves mostly around if they execute in the same process context, if they use a communication mechanism typically used for inter-process communication and if you need to have knowledge of the implementation to reproduce the datastructures used in the communication.
For using a published web-API, all of these indicators point to independent works.