Sadly neither clearing the land nor using the weed killer will have totally got rid of the marestail. The roots go down a long long way. (I followed one down over 3 metres on pure sand).
Cover with the plastic and then when the weeds begin to lift it, take it off, beat the stalks to bruise them, then weed killer

I would use your SBK weedkiller on it. You would need to lightly crush the growth to bruise it as it has a silica coating and will take up the weedkiller more readily. Several treatments will be needed so perhaps start now. Kurtail Evo is available but very expensive that you would only use if you have a large infestation. Good luck1!

if you have already cleared the top growth of the horsetails spraying SBK will be a waste of time and chemicals. It needs applying to the bruised stems first.

Certainly covering with a heavy duty black plastic will do some good. But remember some weed seeds need light to germinate so when the plastic is removed and you dig the soil they will germinate.

I believe there is a specific treatment against this thug.Have a look on Google. Note that horsetail and mares tail are different. Mares tail is the one that lives in ponds.
When it reappears in spring you'll notice that there are two types of frond. One just has green spiky fattish growths at the top of the stem, like a flower bud and different from the rest and will be first you see in spring. This is the one that has the spores, so cut every instance of it off and dispose of it somewhere it can't germinate. Every one of those you miss means lots more plants...
Its an interesting plant though - very primitive and like the ones in pre history that grew huge, and fossilised to become coal!