Doesn't this penalize the tool though? Knowing the amount of data in a table is still important to determine the type of query, even more to optimize it

Yeah, in general the more data you're able to use (assuming the context window supports it), the better results tend to be. We arrived at the data schema being a good enough compromise at which the benefits outweigh the risks for several use cases. Besides, some data stores that are generated by third-parties actually have common schemas (think Sendgrid / Hubspot activity data), so you're not risking much but potentially gaining a lot of sales ops productivity.

Db schemas sometimes contain important information themselves, as well as proprietary architecture.

I see it same as slack leaking private channel names.