Exactly that. So if you use die and do 24:word seeds yeah maybe due to the unknown bias it is like a perfectly done 17-20 word seed.
You shouldn't be counting in words. Words do not provide security. Entropy does.
In my previous example, every roll produces 1.39 bits of entropy. A 24 words long seed phrase is an encoded version of 256 bits. This means you can produce a secure such phrase by rolling said dice for 256 / 1.39 = 185 times. It would provide the same security as to tossing an unbiased coin 256 times, or rolling an unbiased dice for 99 times.