I think it is important to try and understand where your boss was coming from and why they were concerned.
From my perspective this is not about whether you should have "lied' to the client, but more about how you could have steered the conversation back onto the matter in hand - the prototype you were demonstrating - rather than a more "high risk" topic of conversation.
We tend to view our sales in two different ways - one is "technology-led" where its all about the product, and the other is "brand-led" where its all about our people, skills and values. While you may not use these terms, they probably apply.
Technology-led sales are hard to close. They can be long, drawn out processes where the client has a fixed idea of the product (or service) that they try and force into fitting on what you are selling. This can lead to a checklist mentality, which can make people focus on saying "no" as opposed to saying "yes", or the risks associated with the purchase.
It can also lead to what was described to me as "ugly baby" syndrome - if a client dislikes your product on technical grounds, you offer to endlessly modify and customise it, creating a very high cost of sale.
Brand-led sales tend to be easier to close; they are often based on peer group recommendation. Who else uses your product/service?" is a common question in this situation. When people want to own part of your brand, they usually look for ways of saying "yes" rather than saying "no."
Note that brand and technology are both important, but what you lead with sets the tone for the conversation.
In responding to the client's question, you took a "technology-led" approach; its hard not to when you are running a technical demonstration of a prototype. (You can also do brand-led demonstrations, where you highlight how the features and benefits reflect the companies thoughts and thinking; there's one technology company in particular that is very good at this.)
Technology-led is always a higher risk; in this case, I suspect that there was a concern from your boss that you were highlighting that you were not "at the leading edge" as you focussed on the older technology in your response. You also don't know the client's opinion of the techologies in question, so again there is a risk.
I'd also be careful of the "casual question" - if you ever saw the TV show Culumbo you'll understand why. Some people adopt this as a tactic.
Depending on your company, a "brand/people" led response might have been:
"We have huge experience with technology X, which underpins this, and
I know the basics of technology "Y" already. I find this kind of
transition very easy - its essentially part of what we do - and we
would be able to include technology Y if it was a strong requirement."
Its quite a bland statement, but it could move the conversation back on track to the prototype you were demonstrating.
The risk of a brand-led sale is over selling - you win the contract based on brand alone, and then cannot deliver on the technical requirements.
The key thing I would suggest is that in future, that meeting with your boss to discuss the 'selling strategy' for a particular client ahead of time is a good move.
You need to understand the main features, advantages and benefits of what you are trying to sell, have identified the risks the client may worried about and developed agreed responses to these that you are both comfortable with.