Venerable Anandajoti (who I used the pali text of) renders it like this
suññāgāragato vā, nisīdati.
or has gone to an empty place, sits down.
Pallaṅkaṁ ābhujitvā, ujuṁ kāyaṁ paṇidhāya,
After folding his legs crosswise, setting his body straight,
parimukhaṁ satiṁ upaṭṭhapetvā,
and establishing mindfulness at the front,
I doubt the Buddha actually meant only one specific posture, it seams strange considering he never mentions how to stand, walk, or laydown in instructions, he only mentions to use these postures.
but just to note there is mention of how the Buddha lay down, walked... but to assume he was strict regarding posture when there are reasons not to adopt certain sitting postures seams to tight when it comes to here.
Actually just to contrast this with the precepts.
With the precepts the Buddha is very clear as to what is meant, and expands, rewords, incorporates allowances... for the rules so they can be understood exactly what is meant. when a rule says don't wear jewellery, he means it, it isn't a case that he means exept in X, Y, or Z when he didn't say that. and the same is true elsewhere the Buddha says what he means sometimes a general principle can be applied, at another time a literal principle is meant.
so in essance you have to decide if it is a general or litteral principle that is needed to understand these things.
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He who knows only his own side of the case knows little of that. His reasons may be good, and no one may have been able to refute them.
But if he is equally unable to refute the reasons on the opposite side, if he does not so much as know what they are, he has no ground for preferring either opinion …
He must be able to hear them from persons who actually believe them … he must know them in their most plausible and persuasive form.John Stuart Mill