Pissed at interviewers going back on their promise - what to do?

Classic company promised me one thing but they went back on it. Looking for advice on how to proceed from here.

I managed to get an onsite interview with this one fund that started up recently and looking to add a quant-like person. The role is pretty open-ended and it seemed like they were looking for someone who can pitch them a vision. 

The founding members of the team are all pretty experienced and the senior members seemed pretty smart as well.

During the onsite, I laid out my plan + shared some concerns I want to address to see how realistic it is. At the end of the interview one of the heads of the group and I discussed how to quantify my value add, where to start, how long would projects take, etc... I was also asked to share my perspectives with the other head of the group on a later date. 

So naturally I was expecting to hear back. I followed up a week later. No response. Next week I follow up again. Another week later I get a boilerplate rejection email.

At this moment I found out that they made a new LinkedIn post for a job I was interviewing for week and a half before, soon before my 2nd follow up. I looked up the job descriptions and some of it seems vaguely related to the ideas I brought up.

Not accusing them of anything but I am very upset about this situation and looking for advices. What's the best way to follow up with them? I actually liked the team so don't want to burn bridges but at the same time I'd like some explanation. This just feels like a weak move on their part. Or perhaps there was something I could've done differently.