I think the point is the contrast between the ascetic practices - trying to renounce all pleasure - and the pleasure of absorption.
In MN36: SuttaCentral Saccaka asks the Buddha:
“Surely you must have had feelings so pleasant or so painful that they could occupy your mind?”
And the Buddha describes his training with his teachers:
Then it occurred to me, ‘This teaching doesn’t lead to disillusionment, dispassion, cessation, peace, insight, awakening, and extinguishment. It only leads as far as rebirth in the dimension of nothingness.’ Realizing that this teaching was inadequate, I left disappointed.
And then strict ascetic practices. And finally he realises that he does not have to fear the pleasure of absorption:
Then I thought, ‘Whatever ascetics and brahmins have experienced painful, sharp, severe, acute feelings due to overexertion—whether in the past, future, or present—this is as far as it goes, no-one has done more than this. But I have not achieved any superhuman distinction in knowledge and vision worthy of the noble ones by this severe, grueling work. Could there be another path to awakening?’
Then it occurred to me, ‘I recall sitting in the cool shade of the rose-apple tree while my father the Sakyan was off working. Quite secluded from sensual pleasures, secluded from unskillful qualities, I entered and remained in the first absorption, which has the rapture and bliss born of seclusion, while placing the mind and keeping it connected. Could that be the path to awakening?’
Stemming from that memory came the realization: ‘That is the path to awakening!’
Then it occurred to me, ‘Why am I afraid of that pleasure, for it has nothing to do with sensual pleasures or unskillful qualities?’ Then I thought, ‘I’m not afraid of that pleasure, for it has nothing to do with sensual pleasures or unskillful qualities.’
After eating solid food and gathering my strength, quite secluded from sensual pleasures, secluded from unskillful qualities, I entered and remained in the first absorption, which has the rapture and bliss born of seclusion, while placing the mind and keeping it connected. But even such pleasant feeling did not occupy my mind.
Perhaps when practising with his teachers, there was still a somewhat ascetic idea of steering clear of any pleasure, even the pleasure of absorption.