Right, but the XR production line is still going on and made products at a high rate for many years. The Mini NEVER had that type of volume. Also, component costs (like LCD and the camera) are much cheaper than what's in the Mini.
People have always assumed that SE means "small" but that's only because the older designs were relatively smaller phones compared to the newer versions. SE is designed primarily to be a lower-cost, yet completely useable iPhone and the XR is already at $499 with multiple colors (and you have to assume they're making money at that price.) If Apple can upgrade the internal components to more modern specs (5G, better camera, faster processor) for $399, that's where they're going to go.
The current Mini starts at $699 (subsidized). I can't see them dropping that to $399 with the current components so quickly. They would have to completely engineer a new phone to get a lower cost for a smaller size that seems to have limited appeal, and that just doesn't add up.
If you want a small phone, you better get the current Mini or next year's model. Because I think by 2023, the Mini will be gone, along with the older (original) iPhone design with the Home button and large bezels.