While to be pedantically correct they don't literally own the IP, the startup is correct in telling investors that they have the rights to build a business from what software they have. Your license is very permissive and grants them this.
If you don't want people to use your software to build a business, you should release it under a license that forbids it. (There isn't such a license but AGPL effectively accomplishes this.) If you want people to be required to mention you when they use your software, you should release it under a license that requires this. (For example, the unpopular 4-clause BSD license requires this.)
With all that said, it's still kind of a jerk move on the startup's part for them not to even contact you. But modern software is made up of many people's open source software and most of them never get any acknowledgement...
If you don't want people to use your software to build a business, you should release it under a license that forbids it. (There isn't such a license but AGPL effectively accomplishes this.) If you want people to be required to mention you when they use your software, you should release it under a license that requires this. (For example, the unpopular 4-clause BSD license requires this.)
With all that said, it's still kind of a jerk move on the startup's part for them not to even contact you. But modern software is made up of many people's open source software and most of them never get any acknowledgement...