I'm surprised nobody more knowledgable than I has come along...
I recall this has come up in discussion before. The hole definitely weakens the mast. I have seen people suggest welding the door shut; your idea of welding a plate looks better. I think previously, I've seen a suggestion to not use that portion of the tube, but as it's reported being 3m up an 11m mast, that wouldn't leave so very much!
I suspect it partly comes down to how good a weld you can achieve, and whether the welding heat would affect the temper of the alloy; I'm afraid I'm no metallurgist, and all my own welding has been to steel, and of "agricultural" quality!
As my need would be for a longer mast than 11m, if I were to use this, I would be using it as a top on to a probably wood lower section. I would consider lengthening the wooden plug to extend beyond the opening. Heavy, yes, but the weight is relatively low. I won't though. I'm not 100% convinced about all the details in the listing (eg wall thickness - at top or at the base?). It's a few hundred miles from me, so a long way to go to inspect it, and a long way to arrange transport for it.
But it's a good price...