Honestly, I think you're misremembering, which is totally fair tbh, it's been a while and you were a kid!

Since I'm a loser with nothing better to do on a Saturday, I did a little research by watching LEGO commercials from the 80s and 90s, and there are dozens of depictions of boys playing with LEGO, and the relatively rare depictions of girls playing with them are for sets clearly designed for girls, such as Belville and Paradisa.

If you check out this video from around 2:36 onwards, you can really clearly see the differences in how LEGO was marketed to girls, and to boys.

There's a whole jingle that went across the decades with the phrase "he's a lego maniac", and most LEGO sets fall into traditional "boys toys" categories (trains, police, knights, etc.). Even ads for the generic core sets with just the simple colored blocks depicted boys playing with them.

Interestingly, though, almost all of the Duplo commercials depict both boys and girls playing with the toys together, but even those sets are quite gendered - doll houses for girls, for example.

I don't think I've seen a toy advert for a good 15 years or so, except for the infamous a man has fallen into the river in LEGO city commercial, I honestly have no real idea what they're like now, but they were very gendered back in the day for sure!