No, "proportional" just means the two things correspond in size, not that the sizes are equal. In other words there is a positive correlation between the values.
According to the Wikipedia page for proportionality[1], the values "can also be viewed as a two-variable linear equation with a y-intercept of 0". Direct (as opposed to inverse) proportionality is a type of positive correlation but they are not the same thing. Because we are dealing with probability percentage as a proportion of the company's size either the size is greater than zero and the probability is 100% or both values are zero.
So, I think you're confusing 'probability' with 'cardinality of the set of successful outcomes' (let's call that k). If k is proportional to the size of the company (n, say), then you are absolutely right that k/n (i.e. the probability) will not change as n changes. But the statement actually says that the _probability_ is proportional to n, not k. So no, the probability is not constant.
The idea of the probability being directly proportional to n is not technically correct for other reasons (with large enough n, this would allow for probabilities larger than 1), but not for the reasons you're mentioning here.
Perhaps more importantly, the meaning of the comment is independent from this pedantry and should be interpreted less literally so we can focus on its actual meaning.
I don't think "100%" and "probability" can exist in the same sentence (speaking in binary but also saying 0.999)
I think the parent was inferring that there was a chance progress could be reversed at his/her past companies because of the office politics/territory/decorum mentioned in the original article.
Raw efficiency averted by someone's need for the spotlight.