Generally speaking, a manufacturer is free to reverse engineer a product to insure compatibility with another without any infringement. Tamron, and other lens smiths do this with no regard to Canon's patents nor via any licensing fees.
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You are confusing patents with copyrights. You can reverse-engineer, say, the copyrighted firmware of the IBM PC BIOS, and create a "clean-room" development of software that performs the same functions; this does not infringe the original copyright.
You cannot, however, do that with patents. A patent covers any implementation of the patented technology, irrespective of whether it is independently developed. In the scenario you propose, the third-party developers would be liable for substantial damages for patent infringement.
On the other hand, patents expire (IIRC) after 17 years. The EOS mount has been in production for around 21 years, so any basic patents on the technology have expired. There may be patents for subsequent improvements that are still in force.
I do not know if there in fact are or have been any patents on the EOS interface; the forgoing is the situation in abstract, if there are in fact any patents.
It is also possible for there to be trade-secret aspects of the electrical and protocol interface that are difficult to accurately replicate without inside information supplied under license from Canon.
The forgoing is the situation in the United States. I suspect that things may be different in other areas of the world, such as Europe. I get the impression that there are situations in Europe where one needs to license things that don't need to be licensed in the U.S. (But I have no actual information on that--it's just reading between the lines of various statements I've seen over the years.)
Edit to add: Since one or more lens makers apparently did reverse engineer the EOS mount and produce lenses in the mount, and it appears that no lawsuits ensued, then it is quite possible that there are/were no patents covering that interface. However, it is also possible that Canon merely choose to not enforce their patents, or that quiet behind-the-scenes activities resulted in licensing and payment of royalties without the presence of visible lawsuits. (Significantly, Japan is far, far less litigious than the U.S.)