There are a lot of assumptions baked into your position.
1. The employer pays for your time, not your expertise or output.
It'd only be stealing from the company if the company cares about hours worked over output. If we explore this concept in a theoretical sense it's clear that it doesn't hold up.
You have two candidates
One candidate has 20+ years of experience doing the exact thing you want them doing. This candidate says they'll work for you for $100k/yr, and they'll work 10 hours a week, complete all the relevant tasks, and very very rarely cause catastrophic errors or user-impacting bugs.
The other candidate is fresh out of school and says they'll work 50 hours a week. They'll complete the same amount of work as the first candidate, but they'll write more bugs, there will be more planning mistakes causing feature delays, and there's a reasonable chance of catastrophic failure due to debugging-in-prod shenanigans. They are also asking for $100k/yr.
Which candidate is better? Under the assumption that employers pay for _time_, the second candidate is better, but I'd argue most companies should prefer the first candidate.
2. More hours worked produces more or higher quality output.
There's a reasonable amount of research and practical anecdotes that disputes this recently (see companies that have gone to 32-hour 4-day workweeks with no reduction in productivity). Enough that at least, this point is seriously in doubt.
3. Twitter maybe pays a senior engineer $400k/yr under the expectation of their output for 40 hours, and if they get less it wasn't a fair deal.
This is a reasonable take, but Twitter (like most for-profit companies) theoretically has a performance evaluation system, managers, deadlines, etc. They're paying an engineer some amount of money for some amount of output. If that engineer produces that amount of output, Twitter is happy, the engineer is happy, there's no issue. If the engineer working 20 hours or less per week caused them to not meet their goals, then Twitter has the right to fire that employee. They don't, so that implies that they're happy with the arrangement.
4. The employee's salary is equal to their expected output/profit
If the Labor Theory of Value is correct, then companies derive their profits almost exclusively from the labor of their employees.
In order for an employee to "steal" from a company by under-producing work, they would have to earn more in salary + benefits than they earn the company from their work.
This is necessarily not the case (on average) in a for-profit company, because if the company makes a profit and uses those profits to grow or to issue dividends to shareholders, they have earned "_surplus value_" from the employees' labor (on average).
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In any case, it's not necessarily true that you're wrong, but your comment was fairly dismissive and confrontational. There are a _lot_ of cultural and individual assumptions baked into how we exchange salary/wages for labor, and it's worth examining those before firing off moral judgments at one another for not working hard enough or working too much or whatever.