Here's my thought on why they did it.
1. Exclusive ownership of IP has a lot of value to the investors. The IP can be licensed out and can be used as a competitive advantage. When the company is sold later, IP is an important part of it. Claiming IP ownership of your project increases their funding appealing to the investors. Otherwise, another group can just easily use your open source project to build a competing company.
2. Claiming ownership of the IP gives the appearance of their development prowess of building the project from scratch. It adds to their technical competency to the investors. In reality what they're really good at is taking credit of other people work.
Now what do you want to do? It has been a hurtful experience and you are probably pretty emotional. I would. But time to check emotion at the door and start to play ball.
You being the exclusive ownership of IP has tremendous advantage. See 1.
Tell the investor you own the IP. You were just being philanthropic to open source it; however, you still own and control the IP. Investors really try to avoid tangled IP so they can sell the company in a clean bundle later.
As for the startup, you can sell them a license for a large fee to allow them to close-source their new modification. This essentially let them buy off control of a branching of the project. And investors like it since it gives them control over future IP addition.
You can consult for them, but I think it's a bad idea given how these people have behaved unethically.
You can tell them to stop claiming ownership of the IP and give attribution, and remind them any future changes they make have to be open source as well.
Or if you want to play the long game, you can sit back. Let them develop the company. Later have your lawyer sent them notice that you want to audit all their code to make sure any derivation of the project is properly open sourced.