Importing a single signature software wallet seed into a hardware wallet generates essentially no more security than the original software wallet. Probably less actually, since you now have to trust two environments.
It is not safe not because you now have to trust two environments. It is not safe because you import the seed phrase of an online wallet into a cold wallet is not safe, it has to be generated on the cold wallet to make sure that the seed phrase is completely offline ans not online in any way. But if the cold wallet is truly cold wallet and not connected online but in a way that it is offline and the seed phrase and the keys it generates does not leave the cold wallet. Importing online seed phrase into a cold wallet.