What is your opinion towards merging these tags?
Keep them both as separate tags
While heavily related and often talked about together, they are slightly different things:
However, if a system has a lot of noise, QEC may not work well enough to stop the errors from changing the final result. Similarly, as an example, if a bit flip $\left(X\right)$ code is used and $Z$ errors occur, the system is not fault tolerant to that noise/error.
The fault tolerance threshold is the point where noise is low enough for QEC to be used to make the system fault tolerant.
In this sense, while fault tolerance requires QEC, QEC alone does not make a system fault tolerant, so I believe they are sufficiently different to warrant having separate, non-synonymous tags