Is it possible to bruteforce the seed?
No, it is not true. Both are impossible due to math. The mnemonic is just an encoding of a number that is randomly generated (i.e. just like a private key) so it has the same security properties of that 256-bit number.

If you ignore that it encodes a number, consider this: a seed phrase encoding a 256-bit number needs 24 words in the mnemonic. There are 2048 possible words which means that there are 2048^24 = 2.964... × 10^79 possible mnemonics. There are 2^256 = 1.157... × 10^77 possible 256-bit numbers. There are actually more possible mnemonics than there are values to be encoded in the mnemonic. Thus the mnemonics are just as secure as a randomly generated private key (256-bit number) itself.