Alex could still have qualified if he’d won today in straight sets because there was potential for a 3-way tie between him, Taylor and Daniil on 1-2. Both Taylor and Daniil have one straight sets win at the moment.
Daniil now needs to beat Sinner in straight sets to qualify at all, but if he does that he will also qualify first in the group with Sinner second. If he wins in 3 he would be tied with Taylor, who would qualify ahead of him due to the H2H result.