Not the case.
Visa credit cards ($V) can be used at 44 million merchant locations in more than 200 countries and territories.
Mastercard credit cards ($MA) are accepted at 37 million merchant locations in more than 210 countries and territories.
So they're different, which doesn't matter at all except if you're stuck somewhere!
Odds are both are accepted, but it's worth checking a destination country or area within a country before traveling with just one or the other.
It might surprise that even in popular countries, V/MA might not be the most widely accepted. It's only recently that CB didn't beat both in France, though for a while CB+V was more common on a door than CB+MA, but now a CB logo usually includes both V and MA.
https://histoire.bnpparibas/en/the-carte-bleue-a-french-succ...
Also, this is a good synthesis of the duopoly* problem, perhaps less of an issue (aka lower rates) in EU likely thanks to the nationals like CB:
https://seekingalpha.com/article/4536466-visa-and-mastercard...
* About duopoly: Visa has less than a 50 percent share, MasterCard has less than 40 percent, Discover has 18 percent, and American Express has 15 percent. V and MA acting as if 90% share is most likely "conscious parallelism".
https://en.wikipedia.org/wiki/Tacit_collusion#Conscious_para...
See the duopoly example at the end of the article suggesting a multi-round (long term) Nash equilibrium for parallelism.