This summary is not the rule. The rule text controls.
A lawyer generally can't be both a witness and a trial lawyer in the same case. If a lawyer will likely need to testify about something important, that lawyer usually can't also argue the case in front of the judge or jury. The concern is that mixing these two roles can confuse things and make it hard to tell whether the lawyer is presenting facts or making arguments.
There are three exceptions to this rule. A lawyer can testify and still handle the case if the testimony is about something nobody disputes. A lawyer can also do both if the testimony is only about legal fees or the value of the legal work done on the case. Finally, a lawyer can serve both roles if removing that lawyer from the case would create a serious hardship for the client, such as if no other lawyer could reasonably take over.
This rule only stops the specific lawyer who will testify from acting as the trial lawyer. It does not automatically stop other lawyers at the same law firm from handling the trial, even if their colleague will be a witness. Those other lawyers can still represent the client, as long as doing so doesn't create a conflict of interest under separate rules about client loyalty and confidentiality.