What do you make of this?

I don't believe that your claim is true

Maybe I misinterpreted here but it sounds like you're saying you don't believe in mental stamina limits? Maybe you mean that you don't think rationality requires much brainpower?

but if it is I think LessWrong is doomed as a concept.

I don't think we'd be doomed, and there are a few reasons for that:

1. There are people in existence who really can analyze pretty much constantly. THOSE people would theoretically have a pretty good chance of being rational all the time.

2. People who cannot analyze anywhere near constantly can simply choose their battles. If they're aware of their mental stamina limits, they can work with them. Realizing you don't know stuff and that you don't have enough mental stamina to figure it out right now is kind of sad but it is still perfectly rational, so perhaps rationalists with low mental stamina can still be good rationalists that way.

3. There are things that decrease mental fatigue. For instance, taking 15 minute breaks every 90 minutes (The book "The power of full engagement: manage energy not time" talks about this). We could do experiments on ourselves to find out what other things reduce or prevent mental fatigue. There may be low-hanging apples we're totally unaware of.

One example of a skill that I have taken on on this level is reference class forecasting. If I need to estimate how long something will take, my go-to method is to take the outside view. I am so used to this that it is now the automatic response to questions of estimating times.

Okay, so you've learned to instantly go to a certain method. I can believe that this does not take much brainpower. However, how much brainpower does it take to execute the outside view method, on average, for the types of things you use it for? How many times can you execute the outside view in a day? Have you ever tried to reach your mental stamina limit?

I don't use "brainpower rationing" because I frankly have never felt the need to do so.

Do you ever get home from work and feel relieved that you can relax now, and then do something that's not mentally taxing? Do you ever find that you're starting to hate an activity, and notice you're making more and more mistakes? Do you ever feel lazy and can't be bothered to do anything useful? I bet you do experience mental fatigue but don't recognize it as such. A lot of people just berate themselves for being unproductive, and don't consciously recognize that they've hit a real limit.