I think that this solution will help someone to save a couple of hours that I have already spent on the research.
So, I have the combined table that I get using a query:
SELECT t1.id,t2.id as componentId,t2.component,t2.qty AS available FROM t_bom as t1
LEFT JOIN i_components AS t2 ON t1.componentId = t2.ID
Bulk update via a primary key works fine with single table, but for joined tables you can use the following solution:
{{t_projectBOM.recordUpdates.map(row => Object.assign({'ID':row.componentId,'qty':row.available}))}}