It is poorly worded. It doesn't say you have to be disabled or Impaired to not be able to use it. Although it is in the accessibility booklet, "I was unable to use it because I was late" or "I was unable to use it because I had my eyes shut" would appear, on their wording, to be acceptable! However, I suspect a court would take the view of what 'a reasonable person' would interpret it as, and I think having a disability or impairment would be a reasonable interpretation.
The OP already admits to having said, on record, they were in a rush as their first defence - which potentially completely undermines their case. Unwisely, they tried to retract the comment after realising this was undermined their intended defence (no such thing in practice - it's been said!), but then undermines their case further by saying a train just so happened to arrive and they got on board. This would have
no relevance at all if the OP was genuinely completely unable to use the TVM - this is a non-defence used by people who just can't be bothered to buy before they get on, or don't leave any time to do so - and it was unwise to say this too.
The OP says they have provided evidence that their visual acuity is so bad they can read *no lines of text at all* on an optometrist's chart and cannot see anything close to them, which would render them unable to drive safely as they couldn't then see their own dashboard, even with corrective lenses which make them able to see at distance acceptable to the DVLA. Elsewhere, the OP has a different thread where they were driving a car and parked it at the station.
Overall a big old mess. No paperwork or any correspondence supplied in either thread and my suspicion is Merseyrail think they are calling the OP's bluff. Regrettably, the OP has decided to go to court and asked for no help prior to this, nor have they returned once it seemed clear there were no magic words or get-outs for their case, and strangers were confused and picked holes in the chain of events.