When I look at the libreoffice git repository it seems like if aoo has some bug fix that isn't in libreoffice already then they do pick it up and credit the aoo hacker who fixed it. It doesn't seem to happen the other way around (but aoo uses svn which is a bit harder to quickly grep through). It also looks like there is very little development going on at apache anyway (if you ignore the wiki/website stuff). One or two commits a day. So I assume some libreoffice hacker just goes over the commits every couple of weeks and picks up anything that looks interesting. The LibreOffice developer statistics The Document Foundation publishes also show they still import some of their commits from the former ooo code base in the last year. So in a technical sense I do think they are kind of merged.
Which is a fun document. Apparently you can embed ODF in a PDF file. So if you open that PDF with libreoffice then you can edit the data directly. Nice idea.