That is indeed strange.
This idea of eliminating kamma was an idea even before the Buddha.
There were jain monks who used to practice
for so long, standing or sitting very still. They had the idea
that as the painful feelings grew that by patiently enduring
them that this gradually erases kamma from the past.
So I do not think the idea that one could eradicate all old
“sankhara” is something we would find in the Tipitaka.
Which “sankhara”, from which life? But the result of kamma done one hundred
thousand aeons ago or longer can still appear in this life.
There must be an infinite number of ‘sankhara’. And how many
“sankhara” do we make in just one life?.
Rather it is wisdom and the eight fold path that eradicates the defilements…
One student of Goenka referred me to a talk where Goenka mentions the Udana to justify this idea that sitting “experiencing sankharas” is the path. In this Udana Nandavaggo
The Chapter (including the Discourse) about Nanda
3-1: Kammasuttaṁ (21)
The Discourse about Deeds
aññataro bhikkhu Bhagavato avidūre nisinno hoti,
a certain monk was sitting not far away from the Fortunate One,
pallaṅkaṁ ābhujitvā, ujuṁ kāyaṁ paṇidhāya,
and after folding his legs crosswise, and setting his body straight,
purāṇakammavipākajaṁ dukkhaṁ tibbaṁ kharaṁ kaṭukaṁ vedanaṁ,
° he was bearing painful, sharp, harsh, and bitter feelings, that had arisen as a result of
former (unwholesome) deeds,
adhivāsento sato sampajāno avihaññamāno.
mindfully, with full awareness, and without being troubled
But Goenka doesn’t mention that this was an arahat.
As it says in the Commentary:
.see Arahant Feeling Pain: The Udana and Commentary: chapter 3 Nanda (translation Peter Masefield)
It is said that he, a gentleman (and) a resident of Rajagaha, saw the fault with
samsara upon receiving a shock from the elder Moggallana,
went forth in the Teacher’s presence, cleansed the precepts,
took up, as his meditation subject, the four truths, and then, not
long afterwards reached arahantship after eagerly practising**
*vipassana.At a later stage there arose in him a grating affliction—
he dwelled putting up with it by means of reviewing (same), for
there is known no mental dukkha on the part of those in whom
the asavas have been destroyed, although there is still bodily
dukkha. […] Bom as the ripening of ancient kamma
(puranakammavipakakajam) born as a result of being the ripening
of kamma formerly performed; or alternatively born within the
ripening of ancient kamma, within the collective that forms the
cycle of ripening made up of happiness and dukkha, through
being a part thereof. What is this ?—that which is painful. And
in indicating, by means of this “Born as the ripening of ancient
kamma”, that that affliction has kamma as its origination, he
rejects the possibility of its being born due to sudden attack from
without or seasonal change and so on.That was painful (dukkham). that was impossible to be
tolerated by puthujjanas. [166] Sharp (tippam)'. acute; or
alternatively thick by way of their overpowering occurrence.
Grating(kharam): harsh. Stabbing(katukam): disagreeable. Putting
up with (adhivasento): staying on top of (upari vasento), enduring,
tolerating. Mindful, attentive (sato sampajano): possessing
mindfulness and attending by way of mindfulness and
attentiveness seizing upon that sensation.