I mean that if your paycheck is $10k you'll bring home $15k with this subsidy. But if you get a new job and double your salary to $20k you won't get a subsidy so you're entire income will be $20k. You got a nominal 10k raise but only take home 5k more, hence an effective 50% tax rate through this. If there's an actual income tax you have to pay that gets added to this and if there exist other benefits for poor people like Medicare then you might conceivably be worse off with the $20k paycheck than with the $10k paycheck.

Mathematically, a $10k Progressive Guaranteed Income like you describe plus a 50% tax on income over $20k is exactly the same as a $10k UBI plus a 50% flat tax.