Actually, the slatepack adresses would only be linkable because they come online at the same time on tor. So there is no on-chain address, it is still pure mimble wimble.
In case of one-time-use slatepack addresses, this would not be much of an issue, since after each transaction a new slatepack address is generated.
For permanent accounts it is however an issue since these are as the word says permanent, so likability there is a privacy leak. One option would be to by default only have permanent address online at a time unless the user enables the address or changes this default behavior knowing the risk.