The wording doesn’t prevent this. The committee must be informed of the substitution so that we always know which members are authorized to speak in the committee.
We want to avoid situations where more members than expected attend a meeting and changes take place without our knowledge. Sometimes, members attend by video conference and others do so in person, and we don’t know which members are substitutes. When a substitution takes place, we want to be informed, particularly by the chair.
This doesn’t prevent a substitution from taking place during the meeting. We just need to be informed.