I watched it long time so forgot the exact details. But you are saying he could have had enough money from his cofounders, but that was still after he decided to start drug dealing. So how is that refuting that the initial trigger for his drug making was to make enough money for his treatment?

he declined the offer

Multiple offers, in fact.

One in the very beginning of the show, when Walter’s old friend Elliott offers him a job at his company (that Walter originally created with him, but later quit, and then it ended up turning into a very successful business afterwards). With the explicit mention of their health insurance being able to cover all the costs of his treatment.

Then later in the show, Elliott and his wife straight up offered him money to cover everything, feeling that Walter deserves it (not in the least part, for being an original cofounder who was unlucky and quit right before the company got big).

There were more moments like those that i keep forgetting, but claiming that Walter started manufacturing drugs as some last resort to cover his medical bills is complete revisionism.

I think the point they're making is "Walter White is a well-behaved chemistry teacher who resorts to manufacturing and selling drugs after he gets cancer" would still be true even if he stopped dealing drugs after he was offered money.