Can the Syringer reliably spawn legendary bloatflies?

That shows a legendary bloatfly can appear, but one result doesn’t tell us how often it happens or whether hitting a legendary target caused it. The example involved a legendary super mutant, so the target and the bloatfly’s rarity may have been related, or it may have been chance.

If you want to check, try the Syringer on both regular and legendary enemies and keep track of what appears. The information here isn’t enough to treat it as a reliable way to farm legendary bloatflies.