StS2 was being developed in Unity iirc, and they moved to Godot for that new game (and possibly also future games?).

Don’t know what engine they used for STS, but apparently they were making STS 2 on Unity:

At the time, it said it’d made much of Slay the Spire II in Unity, but would still migrate to a different engine if Unity stuck to its guns.

STS was Java running on libgdx.

STS2 will be C# on Godot.

Based on this: https://caseyyano.com/on-evaluating-godot-b35ea86e8cf4

From the article:

At the time, it said it’d made much of Slay the Spire II in Unity, but would still migrate to a different engine if Unity stuck to its guns.

Perhaps they were saying they started on unity and decided to migrate to Godot instead