Why can قُعُودًا emphasize جَلَسْتُ in جَلَسْتُ قُعُودًا?

Maf'ul Mutlaq: Emphasis, Quality and Number

• because every mansub noun emphasizes a verb
• because it gives the number of sittings
• because it is a synonymous verbal noun meaning sitting and is used only for emphasis here
• because it is the person who sat