You posted a link to the site that states its a grey area, based on exactly that, in a court in pretty sure its your intellectual property rights once a lot is modified
Well, that hasn't been the legal opinion, though it has never been tested in court.
It would be prohibitively expensive to go to a court to get a legal ruling. As far as I can find, it has only been taken as far as getting legal opinions from law centres not a legal ruling from a court.
WP and Thesis engaged in a lot of name calling but it never went so far as WP enforcing their license terms on Thesis with court action because Thesis backed down and started following the GPL. We'll never know if WP was willing to put their money where their mouth was and actually follow through on their threat to sue.
It is WP's platform and it is their TOS and license. All WP Themes and Plugins use the WP platform.
It's like saying it's my video and my intellectual property so YouTube has no right to delete it. While YouTube says as long as you use our platform you need to follow our TOS.
But this is getting a bit far from OP's problem.
There isn't much he can do legally. Even if he had legal grounds since the other site is in India.
As suggested earlier, a stern C&D letter may scare them into stopping.
And developers can use code obfuscation or other mechanisms to make it difficult to resell.
And a DMCA complaint may work even if it doesn't really apply. Often hosts/platforms will err on the side of self preservation and find it easier just to honor the complaint.
Is it the same with HTML bootstrap themes?
No. Only WP because WP is licensed under GPL. If you code your own stand-alone solution it would be yours. If you use a platform that has a different license structure then you would be bound by whatever licensing the platform uses.
As suggested by WP:
"... you might want to consider a non-GPL platform such as Serendipity (BSD licence) instead."
The BSD license is open source (Linux uses it) but it does not require that you share source code or publicly allow others to build on your work. So you can create software and keep it proprietary if you want.