This is probably not fully answerable until archives are opened and so forth, but it's probably worth pointing out that a few days before Johnson visited (on April 9), the negotiations were actually not doing as well as claimed in that video, as attested by the public disagreements between Russia and Ukraine:
April 7 (Reuters) - Russian Foreign Minister Sergei Lavrov said on Thursday that Ukraine had presented Moscow with a draft peace deal containing "unacceptable" elements at variance with a previous agreement, comments that Kyiv dismissed as "pure propaganda".
The Kremlin has said talks with Ukraine are not progressing as rapidly as it would like, and has accused the West of trying to derail negotiations by raising war crimes allegations against Russian troops in Ukraine, which Moscow denies.
Lavrov said on Thursday [April 7] that Ukraine had presented a draft agreement on Wednesday that deviated from proposals both sides' negotiators had agreed on.
Ukraine's new draft, according to Lavrov, said the status of Crimea, which Moscow annexed from Ukraine in 2014, should be raised at a meeting between the two countries' presidents.
It also said Ukraine could hold military drills with foreign countries without receiving Russia's permission, something Moscow disagrees with.
"Such inability to agree once again highlights Kyiv's true intentions, its position of drawing out and even undermining the talks by moving away from the understandings reached," Lavrov said, adding that the proposals were "unacceptable". [...]
Presidential adviser Mykhaylo Podolyak, one of Ukraine's negotiators, said Lavrov's comments should be seen as a tactic to undermine Ukraine or divert attention from war crime accusations against Russian troops.
"To make any changes to our position to weaken it would be pointless," he said in written comments to Reuters.
Podolyak also accused Lavrov of interfering with the negotiation process (presumably on the basis that FM Lavrov was not formally Russia's negotiator in those talks--one of Putin's advisers was IIRC.)
So yeah, Lavrov had already accused Ukraine of "deviationism" from the (alleged) deal a few days before Johnson actually visited. So there wasn't really a deal at that point (anymore, if there ever was one in the days before--all I can tell is that there were competing drafts). It would be of course interesting to see (in their entirety) the various drafts that Russia and Ukraine both advanced in those talks, and how they differed, but I think we'll have to wait decades for that.
As for Zelensky's [earlier] public statements, the Guardian summarized them on Mar 28
He used a video interview with independent Russian media outlets to signal his willingness to discuss the idea of Ukraine adopting a “neutral status”, and also make compromises about the status of the eastern Donbas region, in order to secure a peace agreement with Russia. But he said he was not willing to discuss demilitarisation, and said Ukrainians would need to vote in a referendum to approve their country adopting a neutral status.
I couldn't quickly find out what the Russians said on the referendum part(s), but judging by the Chinese press commentary (which has higher google juice), it probably wasn't good:
But Chinese observers pointed out referendums can hardly solve the sticking points in the conflicts when Ukraine and Russia are entering another round of talks.
I don't know [from the available sources] if Ukraine (really) dropped those referendum demands in the first week of April.
There is however this odd piece of news that Russia apparently banned Zelensky's interview in which he said that...
The issue of Ukraine's "neutrality", one of the central points of negotiations with Russia to end the conflict, is being "studied in-depth", Zelenskyy said on Sunday [Mar 27], during an interview with a group of independent Russian journalists.
Zelenskyy told reporters that the issue of neutrality – and agreeing to stay out of NATO – should be put to Ukrainian voters in a referendum after Russian troops withdraw. Zelenskyy said any vote could take place within a few months of the troops leaving.
Russian authorities quickly moved to ban the interview from being published. Roskomnadzor, which regulates communications for Moscow, issued the ban, saying there could be action taken against the Russian media outlets that took part, which included “those that are foreign media outlets acting as foreign agents."
Russia-based outlets appeared to comply with the ban although the interview was published abroad.