The context here is salting so it would likely be omitting prior employment or education/training. I personally think this is completely reasonable in all cases.
Non-salting example: over a decade ago I omitted an entire year of professional experience as a PHP developer when trying to get hired as a Python engineer, because I didn’t want to get pigeon-holed as “PHP dev who can maintain our crummy legacy PHP codebase nobody wants to touch”.
Anyway it would be extremely problematic if employers were entitled to full and complete honesty from applicants but had no equivalent obligations from their side. If businesses had the choice they’d pick the status quo over mutual transparency.
I lied about my potential value (by understating it) to an employer for (longer term) personal and professional gain.
Going back to Union Salting:
Often times the "salt" is a star employee; they're always on time, never say no to a job, pick up shifts nobody wants to take to ingratiate themselves both to management and their colleagues. They don't ask for raises and never complain to management. Their intention is to organize workers and so they want to be the sort of model employee a manager will keep around.
The reason why this practice is allowed is because its illegal for unions to walk into an establishment and talk directly with employees about organizing while they're "on the clock" and on premise.
The parent didn't think they were irrelevant at all, they thought it might signal to the company that it could extract more value by assigning them work they didn't want to do.
Well intentioned hiring managers and teams lie all the time. If they let the truth flow they wouldn't be able to do their jobs. These people don't even mean to lie.
Non-salting example: over a decade ago I omitted an entire year of professional experience as a PHP developer when trying to get hired as a Python engineer, because I didn’t want to get pigeon-holed as “PHP dev who can maintain our crummy legacy PHP codebase nobody wants to touch”.
Anyway it would be extremely problematic if employers were entitled to full and complete honesty from applicants but had no equivalent obligations from their side. If businesses had the choice they’d pick the status quo over mutual transparency.