Doesn't sound to me like they're questioning the plan at all. If you read the piece thoroughly, the thesis seems to be that the Sixers' plan still makes sense, they simply haven't enjoyed the good luck yet that is necessary for all rebuilds to get off the ground.
This, the piece argues, is revealing which fans are genuinely committed to "the process," and which fans saw tanking as a "get rich quick" scheme.
Yes, Sixers fans are having some second thoughts about Okafor versus Porzingis, and yes, like many fanbases, they wish their team had taken Giannis in 2013.
The final line that you bolded sounds more like a challenge to fans, than a challenge to Sixers management: How dedicated are you? How strong is your conviction? How far are you willing to go?
For better or for worse, many Sixers fans have convinced themselves that this is the only credible path for their team to take to acquire transcendent players and reach the promised land. They've sold themselves the story that the NBA is a rigged game and their downtrodden franchise in the tiny backwater of Philadelphia is forced to resort to tanking in order to have a chance at contention.
But hey, I guess if the high water mark for my team over the last 15 years was making the second round because Derrick Rose blew out his knee, I might believe that story, too.