True, it did guess by brute force. But at the time it was a very good machine as it was orders of magnitude faster than a human doing the work. It did reduce the amount of work required, as it was able to take a message that was encrypted and by brute force compare it to a known plaintext message fragment that was always present, the headers were consistent across all message types. A common fault with all crypto in that if you know a portion of plaintext you reduce the search space considerably by only looking for keys that produce that output. That, along with the machine being unable to output the same letter or number as encrypted output ( was not possible to do so by the design) reduced the search space by 1/40 and gave a small margin for sanity checking the output, along with getting captured machines to determine the exact operating methods.
70 odd years later and Moore's law has not done much to reduce that, the computers got faster and the crypto got longer, so both ends kept up.