Do you have a source? That’s a bold claim and is something that would be easily detectable with a signal sniffer if it were true.

in fact, they use separate low power hardware with the only purpose of listening to the “hotwords” needed to wake up the main app. that’s one of the reasons why on certain older phones you cannot activate google assistant while the screen is off.

while the screen is on? all bets are off, but they probably wouldn’t want to waste battery time when a lower powered option is available.