That article at ThoughtCo is, well, should we say … inconsistent?
The statement at the top of the article (the one that you have provided as a quotation) is clearly incorrect as I could think of other examples where uno is used, such as the one that @morbrorper pointed out which is directly related to the phonetics of tsunami, but also for words beginning with “gn” and “x” for example.
If you read further down the page in that article though, it then goes and states a number of other examples where uno is used including -
uno yogurt
uno gnomo
uno psicologo
uno xenofobo
It reminds me of a nice video I saw recently from Davide at Podcast Italiano, where he discusses some of the differences between Italian and Spanish. The URL link for the video should start at the point where he discusses the differences in the definite and indefinite articles, which he considers to be a situation where Spanish is easier -