I argue that as the source of all current claims of the Jutsu, Tomi has the means to discover/learn how to counter them.
You 'made' the jutsu in SL, but that doesn't automatically grant you the knowledge of other users' seals. Sure, I would say that after studying and breaking them down that you may be able to learn the secrets of the seal, but in no way would you be able to look at the seal, touch it and then like an Elder Scroll know all about it.
The Uzumaki peeps are great with seals and logically could break down the FTG seal after some time and learn how to use it, but in order to stop it spreading to everyone, it's not allowed. If you're able to break down the seals of others it follows suit that they could break down a user's seal and then learn how to use FTG without being taught.
Canonically, the 2nd Hokage, the creator of the technique was able to gain access to the 4th's markers to use for himself.
Arguably they used the same seal. No reason that Minato would devise a completely new seal for the technique.
TL;DR
- 'Making' it doesn't allow automatic knowledge on how to break down the seals of others.
- Claiming that after some time you could break it down would only be fair that others could too.
- The 2nd and 4th probably use the same seal. No reason they wouldn't.