It wasn't premature to put him as favourite. The data at the time pointed to that outcome, so he was. If the data moves to indicate an increase of support for Long-Bailey then she'll be considered the favourite. If it changes again then someone else will be.
I don't think either the early YouGov poll or this Survation one are going to look anything like the final numbers, but I'd speculate that a poll of people taken from the subscriber database of Labourlist.org (while undoubtedly not a small number), is not automatically going to tally with the whole membership.