These people won't be spending 30% of their time coding, they will be spending 80% of it coding... and 20% helping everyone else out
Oftentimes companies think they need a "Tech Lead", when what they actually need is a really good Technical Writer who understands code. A lot of time (80 percent) spent "coding" isn't necessarily great / productive / effective if it's just creating more piles of code. But coding good documentation, or refactoring documentation around even mediocre code? That's the quickest route to making the sum of the developers' lives easier.
I've found that spending a good chunk of my time documenting things has paid off in spades in for my team. Writing out common processes (what is our particular flavor of gitflow, how do we use it with Github, how are issues tracked, etc.) and common interfaces (what do you pass to this endpoint to get what results) makes every single person on the team function better.
It also reduces the amount of time I have to spend answering questions--link to the wiki and move on.
Now, actually getting others to buy-in to this is a different story: they don't usually see the point in working to sow the same sort of benefits they reap on a daily basis.
I think a good lead needs to constantly strive to spread knowledge about the work as widely as possible.
If time spent "coding" is just creating piles of code you are doing it wrong. Coding can involve removing and restructuring efforts as well. But more importantly, having skilled programmers working in the code base is one of the greatest ways to drive architecture and establish patterns for the entire team. This is very important.
I do agree that documentation is important, but just felt you really downplayed how large a role an architect/tech lead that actually writes a lot of code can play.
Oftentimes companies think they need a "Tech Lead", when what they actually need is a really good Technical Writer who understands code. A lot of time (80 percent) spent "coding" isn't necessarily great / productive / effective if it's just creating more piles of code. But coding good documentation, or refactoring documentation around even mediocre code? That's the quickest route to making the sum of the developers' lives easier.