When Michael Ventris was working by himself he published junk. A basically crackpot theory that was immediately debunked that Linear B was Etruscan. Then Ventris worked hard to become an insider.
Many key observations for the decoding were done by someone else, a classicist, Alice Kober, right before her untimely death. She worked for 20 years on Linear B and put down all of the foundations. The fact that Linear B has grammatical roots and suffixes, the language is inflected, has case, gender, etc. Kober was one of the first people to work systematically finding patterns and documenting her methods. The work Ventirs did would have been impossible without Kober's methods: extending her work is what worked and gave Ventris his main idea.
Ventris briefly worked with Kober. It didn't go well. But over time Ventris came to know the key players and to be accepted in the inner circle. One of these players, Emmett Bennett, gave him what Kober did not have: the Pylos tablets. By the time they were published she had died.
Ventris extended Kober's work to the Pylos tablets. Her work focused on systematically analyzing groups of characters. When he looked at the results, he made his first critical observation: some groups were unique to the Knossos tablets and others were unique to the Pylos tablets. What if these are place names?
There aren't that many places to be had on Knossos and he knew the Greek names. So he looked for possible combinations and used them to guide the decoding. He used Kober's work and the place names, along with help from at least Bennett, to build a rough mapping from some signs to sound. And then he made his second critical observation: what if Linear B is Greek? Since the Greek names for places seemed to appear.
Then he could try to decode word after word. And along the way he made his third critical observation: many Myceanean scribes were incredibly sloppy spellers. We can even tell now that some were much better than others, but everything is very messy because even the basic rules of spelling weren't agreed on yet. Not only were characters missing, but a single character could be one of 30+ different syllables at times. Bare statistical methods alone often resulted in a mess because of this.
Only small parts of the text could potentially be decoded at this point. None of the classicists that Ventris normally talked to were convinced.
That's when John Chadwick, a linguist, heard about Ventris and tried his idea out. Chadwick was an expert in very old Greek, 1000 years older than Plato. Chadwick was quickly convinced by Ventris because while the decodings were very poor for someone who knew classical Greek, they made a lot more sense to him. They worked together for several years to fix up the decoding.
An architect did contribute the main idea for the decoding, but an architect that was a connected insider, with a background in Greek and Latin, who had published in the area before, knowledgeable in all of the latest methods, with access to privileged information, in conversation with the experts.
The way you put it, it sounds like some random architect somewhere looked at Linear B, worked hard on their own, and came up with the answer. That's not even remotely true.
