So the question is "What should I do?"
You should do whatever helps you to sleep at night. If that's counting/not your 401k, then so be it.
If it were me, I would certainly include the 401k in my accounting. If I wanted to be conservative, I would do it by under-estimating investment growth, or over-estimating my retirement expenses, or over-estimating my future tax burden (which is really just part of expenses). IOW, I'd be conservative in my estimations of the unknowable-future parts of the FIRE calculus.
I wouldn't just ignore an asset for which I know the exact present-value. There's a difference between being conservative and being willfully blind, and I feel like ignoring the 401k is closer to the latter of the two.
I'm also a believer that we improve on the things that we measure (and don't improve on the things that we don't measure). If I didn't include the 401k in my accounting, I'd be afraid that I'd be less inclined to contribute to it. And that (at least for the way I'm planning my FIRE), would be a mistake.