Have in mind, Open Source, especially when you are aware of the topic, means FOSS. I had this discussion on twitter just the past week
It's simple, if you mean FOSS, say FOSS.
When you ignore the ethical intention behind it, those terms are in practice synonymous. The term FOSS/FLOSS was created strictly because "free software" and "open source" describe exactly the same thing, to be able to use it without explicitly standing on any side of that moral battle.
The problem with "Open Source" is that we can't know what the author meant by it. It's not a question of ethics at this point, it's a question of precise language. That he used "freely licensed assets", which could mean any number of things, makes it obvious that he was just very sloppy.
He could have used any of the definitions of FLOSS that are available and at least "copyleft or public domain" for the assets or whatever. He didn't. We can't know what he meant.
The term "Open Source" is very clearly defined: https://opensource.org/osd
Unless you have some reason to assume that somebody is using this term mistakenly (which may happen with any term), there's no problem with it.
Actually, "copyleft or public domain" is a bad term, because it would exclude things like Godot Engine, which is neither copyleft nor public domain (it's MIT). Don't use terms like that, use the clearly defined ones like "open source", "free software" or "FLOSS" - they all mean the same.
How can you be sure he meant the OSI approved licenses? He clarified his 'freely licensed', quite possibly he was sloppy here as well and meant FSF approved licenses, or either, or something else entirely.
FOSS is FOSS, FOSS is dominant, and disambiguates from "open-source" we are pro FOSS. FOSS is boss.
You are really trying to say that Gardiner, someone articulated in the topic, would say Open Source so naively to the point to mean simply "we have access to the source code"?
It's clear that the jam uses "Open Source" as the definition of the Free Software Foundation. And Aseprite doesn't fit this definition.