Question about vapour barrier membrane...?

[KemppiFrog]

Local Brit builder (who knows his stuff) is renovating his own old house. Due to work....this is taking a while ( think cobbler's daughter's shoes) Anyway, he planked the roof and put the membrane on, then left it for a whole winter. No water ingress or damp. Just one or two points where the edges frayed from wind. Put the tiles on the following summer. I was quite impressed.