The web-browser game was copyrighted the moment it was created by the individual. It did not need a copyright notice to be copyrighted, or a TOS to inform you of copyright. The individual was fully within the law to claim infringement and demand you stop disturbing your mod as an unauthorized change to their game. They should have had a TOS up when they started distributing the game, but that's their choice.
The developer could - or have their attorney - send their own legal "takedown notice" of infringement directly to you rather than Tweet about it or approach Google.
Google's takedown and your "copyright strike" is different than your original copyright violation and involves Google's TOS - Google Chrome Web Store Developer Agreement | Google Chrome - regarding copyright violations of apps and code under distribution. By your use of the store, Google reserves the right to remove your extensions when there are
...violations of intellectual property rights, including patent,
copyright, trademark, trade secret, or other proprietary right of any
And, the original game being free and your mod being free rarely matters in copyright law and in cases of infringement. That's something a court would decide on the case and in looking at precedent(s).
...can I get into legal trouble for this?
You could be sued by the maker of the game in civil court; that's up to them. Google's recourse is to simply remove the mod from the store and possibly restrict your use of their service(s), according to their TOS.
For more background, see the Law SE Meta post I have a question about copyright. What should I read before I ask it? .