This isn't what they meant apparently but in a very literal sense, pay can never be enough because it is economically necessary to underpay workers relative to what they contribute because profit by definition can only exist from surplus value, i.e. making more money from selling the product of labor than you pay for that labor.
If I pay a worker $20 per hour to make 20 doodads an hour in my doodad factory and then sell those doodads for $5 a piece, I make $5 for every $1 pay my worker. Of course that extra $4 per doodad has to account for the cost of the doodad factory itself (initial investment, maintenance, material) and the literal cost of doing business (taxes, permits, fees, paying an accountant) and of course market fluctuation (warehousing, building a financial buffer to remain solvent if sales lapse or production has to stop) but if the business is meant to be successful, that needs to add up to less than the extra $4 I'm making. And the bigger that difference is, the more profitable my company becomes and the more spare money I have to expand my business and get ahead of my competition up to the point where I can sell shares in my business to other people and pay dividends from my profits to them.
So in other words not only can't you pay workers exactly "enough" (i.e. 100% of the value they contribute to the company) but you're actively incentivized to pay them as little as you can get away with (and perversely, paying them more by reducing your profit margin may actually be worse for them by making your company less competitive and less resilient). This is before we even get into the politics of what work is overvalued or undervalued (e.g. the idea that CEOs should receive massively disproportionate compensation for their labor, which again creates pressure to compensate for this by underpaying lowly workers).
This, by the way, is why collective bargaining matters. No matter how cool and nice your employer is, the market actively incentivizes them to do the least for you they can get away with and punishes them for doing more. So by not taking advantage of collective bargaining (i.e. using the pressure of all employees to your advantage rather than just yours individually) you're leaving money on the table. Note that is still true if your role is proportionally overpaid relative to other workers in your company unless you literally co-own the company.