You're right, it's true - if you're good at competitive coding you'll be good at LC. But that's incidental. You don't even need to be a Div 1 competitor on (say) Codeforces to be great at LC - the bar is incredibly low. Just being average is more than good enough at interviews. Serious competitive programmers aim to solve problems much harder than Leetcode. Just look at some older ICPC and Codejam problems.
There are some people who basically just do competitive programming to be good for interviews, but they don't get very far - not even to div 1. They reach whatever bar is necessary to clear an interview and quit immediately. The people who seriously compete to go for ICPC World Finals or to get far in CodeJam, they aren't really concerned with interviews.
To be honest, TCO being discontinued isn't that big a deal IMO. Maybe I'd feel different if I was a top competitor. However as someone middle of the road, competing is an atrocious experience.
The web portal is awful and barely works. The desktop client looks like it's from the 2000s and also barely works. To my knowledge the system had weird constraints about input size, and instead of reading from stdin like every other serious website they have the weird "write it in a class with this method signature" format.
Personally, the problem quality didn't compare to websites like Atcoder and most Codeforces contests, and i feel like most people (me included) don't enjoy the "hack phase" format. Maybe if i could bring myself to actually compete more often, my mind would change on these things.
There are some people who basically just do competitive programming to be good for interviews, but they don't get very far - not even to div 1. They reach whatever bar is necessary to clear an interview and quit immediately. The people who seriously compete to go for ICPC World Finals or to get far in CodeJam, they aren't really concerned with interviews.