The company seems to assume that the engagement should scale with the active user base, i.e. that more users coming to the site every month must result in more users engaging meaningfully with the site.
... the number of users who engage meaningfully in the site does not increase proportionally.
Is the observation correct?
First, the reported observation may not be true. More users than assumed may actually engage meaningfully with the site depending on the used definition of meaningful engagement. And more users coming to the site may be accompanied by more users leaving the site and the really active user base may not be as large as assumed.
Is it sensible to expect a proportional increase?
Apart from that it makes some sense. A certain percentage of users will have questions that they want to ask and other fractions of users will have answers that they would like to give. That might be independent of the number of users in total and so with increasing numbers of users you may also expect a corresponding increase in activity. Especially if you think that you are not yet in any saturation regime where basically most of the good questions have already been asked or if you think about the system more like a help-desk instead of a knowledge base.
If the expected increase does not happen it may be an indicator that the system doesn't work optimally. For example, answerers could have difficulties finding questions to answer, duplicates finding should require more effort with more questions and the user experience in general could be improvable. I guess, but this is pure speculation, that the company is worried that the system is not optimally working and meaningful engagement that could happen, doesn't. And in a simplified manner they might think that maximal engagement indicates minimal friction in the process.
Why I would not expect a proportional increase?
On the other hand, I would not expect engagement to scale linearly with the number of visitors or active users. If this system is any good, then the benefit would be that with a growing user base and over time you need less direct engagement. Otherwise there wouldn't be a pay-off of building a knowledge base. It depends of course on how meaningful engagement is defined.
See also this answer of Guillaume Cretot-Richert to "The company’s commitment to rebuilding the relationship with you, our community", which basically asks the same as this question and argues that a successful Q&A should see lower and lower levels of direct engagement.