This Troublesome Pair is one I come across constantly. Interestingly, it features just as much in non-native English writing as it does in native English writing – this is usually very much not the case, with the errors found in each kind of writing not overlapping much at all. I can only think that it’s a typing error rather than a misunderstanding or mis-learning.
Then means “at that time”, “after that, next”, and, as an extension of that meaning, “therefore”. “He went to the park, then he went to the shops”. “You have made a mess of the spelling, then you will find people can’t understand you”.
Than is used to introduce the second element forming part of a comparison – “A cat is usually smaller than a dog”. It is used to express a sense of something happening next in a very specific phrase: “No sooner had the company reinvested in stock, than it went into liquidation” and I wonder, on reflection, if that is how the thans have snuck into the place of thens – maybe there’s an echo of this in the writer’s mind, or it’s a case of a rewrite of a sentence that then goes a bit wrong.
Anyway, it’s not strictly necessary to use the no sooner than phrase, so it’s perhaps best to stick to then for times and than for comparisons.