The article describes the issue when trying to add sorting of non-BoostMatching results with BoostMatching filtered results.
The issue is that when adding sort to the results it will be ordered in that sort order only. The "score"-mechanism will be replaced, which makes BoostMatching useless. This is by design.
For more information on Boosting with filters please see here.
It is possible to first order by some property and then by score. This would ensure the boosted document always be at the top. Here are some suggestions on how to accomplish it:
Ordering by term then by relevance
Episerver Find - Conditional Sorting
The problem is to divide the result in two sections. The first section should be ordered by score and the second by some other property. It is possible to divide the results in two sections and then order both by score or something else, but not different ordering types.
The easiest solution is to make two requests and concatenate the result sets.