I think the bayonet was suggested because it may have been broader, studier and double edged. Many bayonets of the period were.
My reading of the evidence is that it is insufficient to conclude a bayonet was used in the stabbing of Martha Tabram.
The soldier as murderer conclusion, while possible, is also uncertain. The only evidence is that one (might I suggest unreliable) witness states Martha was with a soldier much earlier that night and a policeman who challenged a soldier close to the murder scene was told he was waiting for a chum who had gone with a girl. This soldier was early to mid 20s. Describing Martha Tabram as a girl might have been a stretch.
Whether Martha had been drinking with Pearly poll and soldiers earlier or not, and whether Martha had entertained a soldier's friend during the evening, or not. There is no evidence either was the killer.
I am more convinced by Abby's argument that this was an early murder of the ripper, for the reasons she has provided. It would fit with the escalation and refinement of technique and MO as the ripper murders progressed.