The RowNumberColumn documentation https://www.bryntum.com/products/grid/docs/api/Grid/column/RowNumberColumn, when you check, to show only the public member the page, the info became wrong, I guess:
Support Forum
Hey henrique,
Thanks for the report, I created a ticket to fix that https://github.com/bryntum/support/issues/8823
Best regards,
Márcio