My guess is that Lindy wound and tested them on a guitar amp, never realizing that bass amps are different even in the input stage. Their resistance is fairly low and given their overall output, I'll wager their magnets are pretty small too. I've never played a 60's Bird, but the ones from the 70's I've tried were way hella hotter. I'm thinking Lindy got the tone without necessarily copying the pickup closely.
It is weird to hear that Thunderbird snarl and be holding a Les Paul.