http://www.themodernantiquarian.com/img_fullsize/110354.jpg
I note that the dug out tree trunks at Disgwylfa Fawr are not referred to as canoes in the report. As their lengths are only 2.4m and the other 1.08m, they don't really seem the right size to be canoes.
I recall as a wee boy being taken up to Loch Doon in Ayrshire by my dad and uncle. My uncle had trained at the seaplane gunnery school up there in WWII. On the shore was laid out a row of ancient wooden tree trunks. Some were just that, others were obviously dug-out canoes. They were part of a group of canoes (some 7 metres long) found when the loch level was lowered. A couple ended up in the National Museum. The rest were left there. I remember sitting in one with my wee brother and playing!
Then again, as you say, just because they were hollowed out logs doesn't mean they actually were... or were meant to represent.... dug-out canoes. The point is what else could they be or represent?