Is it true that Grappa is the distillate with the highest alcoholic strength on the market?

It is false; it is an image created around grappa by some uninformed writers.

The minimum alcoholic strength at which it can be found on the market is 37.5% vol. which corresponds to the same percentage of ethyl alcohol and is equal to the strength of many other spirits.

 

For those grappas that are entitled to geographical designations (Piemonte, Lombardia, Trentino, Alto Adige, Veneto and Friuli) the minimum alcohol content is 40% vol.