What it means is that they only pay for generation that's renewable and the energy company assures them by contract that they'll only be paying for renewables.

The rest of us, who might be on green tariffs usually get a commitment that on average almost all our energy comes from renewables. When supply is short then I'm the one who's paying for gas generation rather than the company who has committed to greenwashing. But that also drives investment in renewable generation, in the UK renewables have gone from 2% of power generation in 1991 to around 50% now.

No, but they cover their fossil fuel energy usage by entering into PPAs [1] for new clean energy projects that get added to the grid. This allows renewable projects that would otherwise not have been economical to finance to get built. This is additional renewable capacity that that goes into the energy mix. It moves others onto renewable sources, often without them even knowing it.

From a climate change perspective, what happens at one particular moment doesn't matter -- it's CO2 emissions globally over time that do.

[1] https://en.wikipedia.org/wiki/Power_purchase_agreement