Pretty sure that they boost in duos so atleast one of them doesn't use the vpn.
And they probably lie about using vpns cause how can you prove them not using it
Have a friend who was a intern at riot and used to work support tickets they 100% use a vpn and its easy as hell to see riot side if your being boosted safest way if you want it is to just duo boost but even then once they tie that vpn ip to a booster its game over.
the thing is when riot looks at a account they see a list of all ips associated on the account and the providers of those ips so even if x booster is on xvpn they will see you are normally on comcast then for 3 days you where on x proxy and climbed 2 divisions its glaringly obvious when a proxy is used company side. In my job i do much the same thing tracking large fraud with corporations and proxies don't do shit to hide the intent just where the intent is coming from.
Most boosters, unless boosting full time as their source of income/and or if they are extremely high ranked/known in the scene, will rarely ever use a VPN. Those that advertise it, rarely use them.
Boosters who use a VPN, use it to protect themselves, not the client. Unless said booster happens to have a VPN that matches your location/similar IP, it's nearly always for their protection.
I think it all depends on what the volume they boost is. If you do not do a lot of accounts, I don't see why you would use one. If you do many accounts it would make sense not to tie them all together.
Been boosting for years never had a problem with not using a VPN, like @SellerMan said I worked for a website and they told me if the customer asks just tell them that you use a VPN. Either way never had a single customer get banned or infracted because of me boosting without one.