I was responding specifically to your "otherwise you lose the tax advantage" bit, which is still false. The money still grows with the same tax-advantage as a traditional IRA/401k.
Spending the money on medical expenses at 65 will maximize the value (this actually isn't universally true because IRA/401ks have required minimum distributions which can make holding onto the HSA more valuable), but you still get tax benefits by not covering medical expenses with it as long as you wait until 65+.
I also realized that I meant to say "not use your HSA for any medical expenses pre age 65" in my original comment. Sorry for any confusion.