Do the Stetzer plug-ins when adequately applied also defend against PLC/TWACS smart meter transmission from neighboring homes? We are in close proximity to four other homes PLC smart meters Impact other homes as well.

This is not something we have measured.  Different models may operated/behave differently and at different frequencies.  Do you happen to know the frequencies they use for data transmission?  Most likely the meters are producing microsurge electrical pollution (dirty electricity/emi) as they most likely use a switch mode power supply.  The filters will help with that issue.