There are two elements in this post on Reddit
A -: borrowing money to contribute in BVG
B -: not transferring VB funds to new employer
Regarding B -: I think the OP at Reddit didn’t actually say that they are over contributing to BVG by intentionally not declaration VB. The post was talking about not transferring VB which doesn’t automatically mean that they are over contributing to new BVG. This option of not transferring VB to new employer would be closed soon I believe
If they are over contributing to new BVG (by not reducing the potential purchase amount considering VB) then it would be a tax fraud.
Regarding A -: I don’t actually think that this it is any sort of fraud. How someone funds their contribution is not relevant for voluntary contributions. I am not aware of any rule that says that if someone wants to contribute to BVG voluntarily then they cannot have a personal loan. This does sounds like a loophole though.
Another point regarding A. It only works for first year if the intention is to not have more than 15K of rolling debt. The second year onwards, it makes no difference if you borrow more money from bank or take it from your savings account.