This is an OVA. It is actually 'stand alone' and has little to do with the main series. This presents itself as one of those 'ever popular' historical period re-envisioning episodes, and seems to use the 'personalities' (more or less) of the characters at the end of the series- but not really. Long story short, this OVA is constructed like a filler episode, is completely disconnected from the series, and probably isn't worth watching unless you're a completionist.
In addition to the characters being 'off', the plot is weak, and resolved multiple times by what I assume are blatant and intentional Deus ex Machina resolutions.
You my find the nonsensical/quirky outcomes humorous (I think that was the intent), but in comparison to the original series, this episode is a bit of a let-down to say the least.
I don't know why they made this. It's not horrible, but it's not very good either. It was probably created as 'light-hearted', intentional nonsense- but doesn't quite pull it off.
Watch it if you want, just know it presents as a 'proper (but bland) story' for about 2/3rds before spiraling into insanity/obscurity. There is a reason the main series is ranked around 500 (good), and this one ranks in around 1700 (nowhere near as good). The reason is they are two completely different anime.
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Any reviews I write which get a 'helpful' rating under 30% I retire.