Edit: and if they do I expect some solid next gen game footage, they will have to "show not tell" in order to prove Series S is a real next gen leap.
The expectation is that the Series S will be spec'd to run the same games at a lower resolution, so assuming they accomplish that there wouldn't be much to show.
Personally, I'm still not convinced of a couple of things.
1) I'm not convinced Lockhart/Series S comes out before holiday 2021. They will want to focus manufacturing on cranking out as many Series X's as possible, not splitting those precious resources between two models - one of which is essentially a budget option. Plus, it runs the risk of muddying their focused next-gen messaging by immediately having two models with nothing but a letter to distinguish them.
Lockhart was also believed to have been mothballed by Microsoft until roughly fall last year, and I'm not sure how you'd have a console in mothballs ~16-18 months before it's supposed to come out.
2) I'm still not convinced Lockhart is what we think it is. This 4 TFLOPS rumour seems to have taken root, but strikes me as skirting very close to the line in terms of the power required to reliably translate Series X games to something like 1080p or 1440p. There was a more recent leak suggesting it could be closer to 7 TFLOPS, which I think is more realistic.