What is the world's strongest beer?

It's known as "Snake Venom".

At 67.5% ABV [alcohol by volume], Snake Venom is officially the world's strongest beer.

https://www.htfw.com/brewmeister-snake-venom-world-s-strongest-beer-free-branded-glass-beer-lager

Snake Venom is the winner of the title of the strongest beer in the world. The beer comes with a warning because the alcohol level is 67.5%. Yes, you read that right. The beer was born in Aberdeenshire in Scotland. The brewmaster wanted to brew unusual beers, so in 2013 he came up with the Snake Venom. The beer is made with smoked peat malt, two yeasts, beer and champagne. Despite the high alcohol level, you can still taste hints of malt, hops and caramel. The beer style is between a brown beer and a lager. We recommend sharing the beer when tasting, so you will not fall under the table afterwards.

https://beercrush.eu/en/blogs/articles/bieres-plus-fortes-du-monde

The same site as your first link (a store) actually lists a beer with a higher ABV: Beithir Fire 75% ABV World's Strongest Beer

https://www.htfw.com/88-brewery-beithir-fire-scottish-beer-lager

This is no contradiction (and there are more contenders for world's stongest beer) as the title depends on what you allow to count as "beer".

Both of these drinks are actually fortified beers - a normally fermented beer first strengthened by a freezing process (which by itself can reach ABV around 40%) and then blended with distilled spirits - and marketed as such on the label.

Beithir Fire is considered the strongest "beer" in the world. Have a look at some tasting:

https://www.youtube.com/watch?v=h_852FsCvTQ