15A is by no means essential. It's important for hitting breakpoints and thus saves you stardust if not maxing, but the likelihood of a breakpoint being achievable only at level 40 with 15a is very, very small (though it does exist). I have 4 maxed legendaries with less than 15 attack and I don't expect to ever be disadvantaged because of that.
I'm surprised to find that 15d Lugia can indeed do the 1-v-1 even in Cloudy, as reaching the bulkpoint means it takes the exact same amount of damage from Counter and dodged Close Combat as a from a non-boosted Machamp. The difference is indeed huge because the 1-v-1 can not be done against a boosted Champ if your Lugia has less than 15 defense.
The 15a is still preferable. Only go for the 15 defense one if you absolutely want to complete the 1-v-1 and Cloudy is your most common weather type to the point that all other weathers are rare. There is no real prestige to be gained from completing it in Cloudy, but less than 15 defense Lugia can't even try it in Cloudy.
PS. Now I feel extremely stupid because my Lugia has 15 defense and I've been skipping boosted C/CC Machamps for months because I thought it couldn't be done. Never occured to me to check it.