Given that trick is spreading for several sites now, the trick won't last long. Google could for example generate secret unique user agents for the biggest players. Biggest players would then only allow requests from that secret unique UA.
Edit: ah no, they stopped https://developers.google.com/search/docs/advanced/crawling/.... I don't think 2 DNS lookups are acceptable to block a GET request but I think it can be done out of band, i.e. isGooglebotIp function can fire away a Redis query and if nothing was found, to put the ip into the DNS verify queue. A few requests later, the bot will now get banned thanks to a new record in Redis.
Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Firefox/78.0
This method is already used by stackoverflow to hide the sitemap.
I was under the impression that serving other content to google would greatly punish your pagerank and even pull you off the search results completely.