I think Sieliski is exaggerating a bit, for one thing they're competitive right now without Embiid and Simmons, so there isn't the urgency to act as if they're trying, the current team is better than say the Timberwolves with their "non-tank" acquisition of two #1 picks. Rookies like Saric and TLC are getting PT, TJ is blossoming, Nik has found a role, etc. So it's not a wasted season.
With Embiid, they're just being cautious after being blindsided, given how they've handled him, no way he plays on that knee if they thought there was an issue - but that happens at times - players play through aches and pains, and sometimes it's only days later the medical staff finds out it's serious. I expect him back in a couple weeks, but they'll be cautious.
With SImmons, I think they're doing what they did with Embiid, err on the side of caution, it's not like he'd help them win this year, he has a steep learning curve and joining the team at the end of February doesn't give him much time to get up to speed. So at best if they activate him, it's just to give Simmons a taste of NBA play, coming off the bench for a dozen games. The foot is probably 100%, but that's a tricky injjury, and they don't want an Embiid-like setback.
With Jah it's just a matter of holding out until they're offered something of value, ironically this team is now too good to settle for "trash," guys like McDermott who would struggle to find a role (he's not beating out RoCo or Saric, good shooter but bad defender, been there, done that). Doesn't mean BC thinks he's going to get a top 5 pick, but does mean they can get more than they got for Grant. Jah has value for a team like NO where Davis would prefer to play PF and let Jah take the beatings in the low post - but if NO wants them to take Anjica and settle for a protected 2018 pick, why bother? You can wait until this summer and make that deal, and hope something better comes along.