Note the phrasing is "we run paid trials with potential teammates before bringing them on".

While at-will employment does mean you can be fired at any time, the default assumption is that your job will continue, barring something happening (like poor performance, or the company being unprofitable, or your boss being in a bad mood that day).

The phrasing here makes it sound like the default is that it's time-limited and an extended interview. Sure, technically at-will and "a tech interview" both have the same amount of job-security (exactly zero), but there's social expectations around employing and firing people, and overhead for the company, which lead to full-time employment having more security in practice.