Mmm, this might be anecdotally true but more broadly double and triple IPA's are (in theory) double the hops, not necessarily alcohol. It was more of a description of flavor notes than of the content of alcohol, though, so sometimes that included extra fermentation steps and sometimes more alcohol, but always the flavor was intended to be stronger and more hoppy/bitter.
The strength of the beer isn't particularly unique to IPA's (be they single, double, or triple varieties) as someone else pointed out. I'll never forget getting a belgian tripel while traveling in europe, and it completely knocked me on my ass it was so unexpectedly strong. I didn't know a beer could be strong without being undrinkably bitter