It’s an abbreviation.
Not all abbreviations are acronyms, only the ones that take the first letter from each word. Lego takes two letters from each word, so it’s not an acronym.
On a similar note, some but not all acronyms are initialisms, if they’re spoken as the letters rather than the “word” they create.
FYI, DIY, PS are all initialisms, and also acronyms, and also abbreviations. ASAP, SCUBA, and LASER are acronyms and abbreviations, but not initialisms. Lego, appt, and st are all abbreviations but neither acronyms nor initialisms.