They had to build up the resource grade, meaning - increase the...

They had to build up the resource grade, meaning - increase the indicated tonnage. Dont forget, we are in the PFS phase and in any case, an upgrade of the resource is required.
For example, according to the JORC 2012 standard, the PFS has to have an increasing level of geological confidance. There can be no PFS without sufficient quantification of Indicated and measured resources, which are subsequently converted to Probable and Proved reserves at the DFS stage.
To cut a long story short, there will be additional drilling and classification of resources to convert from inferred to indicated and better.
I hope this explains why we need the extra exploration/drilling.
In any case, if I wasn't a holder yet, and I flet this was not to my liking, I simply would not invest in this company - save my money for other better companies.

This is a suggestion to you - try reading up on some basics on the various stages in the life cycle of a mining company. I've included some material that I personally consider quite decent and well researched.

Firstly we have the JORC code 2012: https://www.jorc.org/docs/JORC_code_2012.pdf
Read chapter "Reporting of Mineral Resources" para 20 onwards - it contains a brilliant explanation of why mineral resources upgrades are essential for Pre-Feasibility Studies. The following diagram is paramount to understanding the inter-relationships between resource and reserves


The following links to an article that discusses the 43-101 standard (followed in Canada/USA etc). The information is good though and applies in similar manner to the JORC standard:
https://kuchling.com/four-study-stages-concept-to-feasibility-which-should-we-do/

Guidance for the use of theterms scoping, pre-feasibilityand feasibility studies, crown minerals act 2017:
https://www.nzpam.govt.nz/assets/Uploads/permits/minerals-guidelines/guideline-scoping-pre-feasibility.pdf

Visualsing the life cycle of a mineral discovery:
https://www.visualcapitalist.com/visualizing-the-life-cycle-of-a-mineral-discovery/