I think it feels like a bit of an extrapolation from recent market conditions. But someone who’s 28 is still in their 20s and can have enough experience to be considered a senior engineer. Big tech companies will pay >$250k for that and recently those companies have done very well in stock markets so if you were offered $x of shares (at the current price) over 4 years at the start of the job, they would now be worth say $2x. If you combine that with a job market where those companies are all competing for the same people it is likely one could do well by being in the group that is being competed for.

So I think it is reasonably plausible for someone towards the older end of the range with a bit of skill and good fortune. It feels unlikely that the trend which I described above (of company stock going up 2x every two years) will continue so I would expect effective compensation to go down a bit. But on the other hand compensation can be pretty sticky and plenty of people will be willing to quit after reaching the 4 year cliff (where their stock awards would no longer be worth 2x what they were meant to be worth) to look for high pay elsewhere.