Maybe it's "portable" in the sense that it's just an executable to launch, with no installation needed? I don't think they claim it is multi-platform.

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6581 3 hours ago | parent [-]

"* Also runs on Linux/Mac (including apple silicon) via Wine *"

	▲	Krssst 2 hours ago \| parent [-]
		Their website makes their definition of portable very clear in "What makes a portable app special?". It's quite clear they mean more a (much) stricter variation of the "no installation" definition than the "easily buildable on other OS" definition. Though they do mention execution under translation environments as a requirement.