The larger river rock does not create complete barrier to sunlight as the gravel and pea stone will do. Likewise, the larger river rock will not create a 'blanket' over the earth beneath that will promote and retain the heat of decomposition, which is really what you want to 'compost' the old growth beneath the fabric. Lastly, landscape fabric is either a woven or non-woven material that is not as conducive to promoting and retaing the heat of decomposition by itself as plastic sheeting or layers of newspaper will do (as in 'lasagna gardening' techniques).

Without additional information, I'm guessing that the peastone and gravels were installed over the fabric in sufficient depths and quantities to form that 'blanket' over the earth below, not allowing sunlight to reach it, and keeping the heat below it.

Possibly some seeds have wedged themselves between the stones on top of the fabric. They should be easy to pull.....or spray with a weed killer.

Yes, your landscaper made a mistake in using landscape fabric. Landscape fabric is evil. (Just my own humble opinion, of course.) I don't think there is any way to pick it all out. Spray it with round-up.

I've did the same type of thing early this summer. And had the same results. More recently, I've done a new section and followed all of the tips given above: layers of newspaper for decomposition, then the fabric, then about an inch or more of sand and pea gravel to keep out light, then my decorative rocks. So far so good re weeds and grass!