At the very least, the lefty would have trained against predominantly right-handed fencers, while right-handed fencers also train against predominantly right-handed fencers. The advantage to the lefty is obvious.
What you’re saying doesn’t make any sense — this persistent fraction of lefthandedness goes back 500,000 years. Fencing is a fairly new skill.
Historically, combat was engaged in by amateurs as the occasion demanded.
Even when training started to become available, the likelihood is, a lefty would train righthanded because whomever is teaching him (probably a family member) would be righthanded.