You're right that "Senior" is still mid-level, but the author is being conservative.
The reality at a healthy, profitable company is something like this: $250k being the "total comp" for someone is a T5 seems right to me, even adjusting for risk. A T5 at Google, Facebook, or Apple is likely earning much more than this as their stock from 4 years ago is vesting at a much higher price than it was granted at.
150k guaranteed comp, 25k bonus, 75k vesting equity is low for the companies w/ equity run-ups in the past several years, in my opinion. Also worth noting -- the bonus is basically guaranteed. It can be much higher if you get very good performance ratings.
The author also didn't include 401k matching (about 8.5k there), free offsites to go skiing or go to Vegas (w/ team trips to Hawaii not unheard of for high profile, long-term projects), generous medical/dental/vision benefits, free food, free gyms, matching charitable donations...
And IDK if it's just my team, I don't see people working extremely long hours.
You're totally right that you're at the mercy of stock prices in some regard -- but it works in two directions. Both FB and GOOG have gone up something like 150% in the past 5 years. So if someone got an initial stock grant of 200k vesting over 4 years, last year that 50k at vest time was more like 100k. Meanwhile they've gotten subsequent equity refresh grants -- the actual equity comp might be more like 200k, not 75k.