The only thing that was different between the two versions of the withdrawal agreement (Note, not a trade deal) was that May's deal featured and open ended backstop on Ireland and Boris' featured a fixed term regulatory alignment. As we have a trade deal, neither of these options is active.
Are people really suggesting that Theresa May with all her red lines would have then gone on to negotiate a substantially different trade deal with the EU in the end? We would still be outside the customs union and single market and so really it wouldn't make any difference.
This is just part of the historical revisionism by brexiteers who want to try and pass the buck for their shit decision in pushing for such a hard brexit and failing to build consensus. May or Johnson it doesn't make any difference really to the fact that businesses are now struggling because we are outside the customs union and single market. The only difference is that Boris was delivered with a large majority and a purge of remainers from the Tory party that meant that the passage of this deal and the final trade deal through parliament were much easier and there was zero need for any consensus with any remainers.
So I suppose you could argue that had remainers propped up May then they would have had a minority government that may have had to compromise, but also may (more likely) have just got so bogged down in trying to build a majority for each vote, with the erg holding the cards, that we would have ended up without a trade deal at all.
So, to answer the OP, no, remainers shouldn't supported May's "deal" as it was functionally no different to Boris's and propping up a minority government would have been a big risk in terms of leaving with no deal due to the erg's likely blocking tactics in such a scenario.