To be fair, Labour supporters probably did help Layla Moran win in Oxford West and Abingdon.
However, the party was actively campaigning in a number of seats the Lib Dems wanted to win in fights against the Tories (Southport, Cheadle, Hazel Grove*, Portsmouth South), letting the Tories win in most of them. The last one is a bit of an oddity, in that Labour somehow managed to win the seat, after the Lib Dems basically gave up and voted Lab instead. On the other hand, if they had diverted the man hours spent there to Southampton Itchen they would have won there instead without doubt. There are also seats like St Ives and Richmond Park where Labour voters could have easily kicked out the Tories if they'd actually wanted to, and the way to do that was always going to be with a tactical vote for the Lib Dems. But no, they'd rather be represented by Derek Thomas and Zac Goldsmith...
The problem is that for a lot of people in the Labour party, the main aim seems to be making as many seats as possible into Labour vs Tory fights - in the belief that if they do this then all Lib Dem and Green supporters will then rush out to vote Labour - and if that means handing the Conservatives a dozen seats on a silver platter then so be it.
*admittedly based on the results it's unlikely we'd have won this one anyway.