Yep, that's just a liner (or a trip of an old blanket I think...).
Fleece is the only liner I've used and it's never been a problem. I've boosted here with disanas flannel folding booster, so there are lots of layers of fabric under the fleece.
I think with j, I used to put extra fleece in the middle somewhere if I was boosting heavily when he was older, to try to repel the pee back in. It's been a long while though.
This set up here lasts Percy fine