That does not mean that someone may not be impeached for high crimes and misdemeanors committed before their current public office.
Impeachment is a serious punishment and only used in absolutely necessary situations.
Even One Particular seems to concur.
There is just as much chance that they will NOT be impeached for actions that occur before they took office, so your point is moot.
In fact, during times when impeachment WAS considered for previsou so-called "crimes," they committees decided they could NOT use those as evidence for impeachment, as I already referenced concerning Richard Nixon.