I certainly agree that we shouldn't bank (figuratively or literally) on positive social expectations, such as expecting an employer to treat us fairly.

However, AppSumo is apparently promising the opposite - they are setting the expectation that it is quite likely new hires will not work out and you should be fully prepared for termination.

Now that's an assurance I'd take seriously.

If the company follows normal at-will employment practices, there would be no need to mention it.

The fact that they explicitly call out this practice is probably a sign that they lay off a larger percentage of new hires compared to the average company. Otherwise, why mention it?