As far as I can tell most instances of impute used in this way come from subject matter dealing with law, and seem to have been more common in earlier times. However this usage seems to have carried on through into legal and legislative (parliamentary etc.) contexts and settings up to this day. Also, it seems that some people have adopted this type of use in writing or speaking about general topics, as shown in the stone tools book citation in my question, but there are also other examples I found, such as in Figures of Simplicity: Sensation and Thinking in Kleist and Melville, 2010, a book that analyzes literature from various authors.

I have checked the 1913 Webster's Dictionary and it only lists "impute" as a transitive verb. This is also the case in the 1828 edition of Webster's. I've also checked a law dictionary but its examples show a use which is consistent with all dictionaries, eg.:

The mother's negligence can be imputed to the child in any claim on behalf of the child against the truck driver.
The People's Law Dictionary

Based on what I've seen, this intransitive usage is generally nonstandard, and when seen or heard it's likely said in a legal or parliamentary setting or context. Also, when impute is used in this way it seems to mean something like the intransitive claim or charge. Eg.:

The Treasurer's legal team argues the Fairfax articles, published last May, impute that he is corrupt and accepted bribes,...
Financial Times article

However there are plenty of examples of both individuals, media outlets and writers using it in this way in general terms, though I can't tell how common this is as a proportion of all uses of the word.

Also, I'm not sure if this is of any value, but a Google NGram search gave the following result.