Yes, but web standards were at least an order of magnitude simpler back then.
The DOM, etc through the work that eventually got labeled HTML5 made it possible to follow the spec and, as a super simple and "obvious" example, parse URLs in a way that worked. The existing specs of the era were often incomplete, semi-frequently what they did say did not match how browsers actually worked, and frequently ambiguous which meant that implementing anything required exhaustively testing what everyone else did to try and match it.
TC39 eventually recognized that the same issues existed in the ECMAScript spec, and recognized that ES4 did nothing to resolve those issues, while adding considerable complexity and more of the same issues. That's how we ended up with ES3.1 - it was an attempt to actually correctly and completely specify the language and runtime, ES5 continued/finished that catchup (ES3.1 did not resolve all the issues) while adding a few important features, but was still in the "make it possible for someone to implement the spec and be confident that they could run anything any other engine could run".
So in the modern browser there are many more specs, but those specs are much easier to confidently implement as you can get basic working behaviour by essentially translating the spec language into your environment.
Making the resulting engine fast, memory efficient, etc is of course another thing entirely (although even that is aided by removing ambiguity in the specs as the lack of ambiguity means that you can actually write tests now).
Having said that, Gecko wasn't a Netscape start from scratch effort. They acquired and built on a very simple and modular cross platform rendering engine that was used for the preview function in an HTML authoring product called WebSuite by Digital Style out of San Diego. (This is where Netscape got engineers Rick Gessner, Peter Linss, and manager Jim Hammerly.)
So, maybe if you count the WebSuite and the Netscape efforts both, you get something like 5-6 years of effort to get Gecko up to sufficient capability to compete in the market. That's a far cry from "it took Mozilla 25 years" though.
And yes, the Web is much more capable today, which makes building a rendering engine more difficult for sure, but the 25 year comment seemed wrong enough to me that I thought I'd share some of what I know as a Mozilla participant for the last 25 years (and a full-time Netscape/Mozilla employee for the last 22.5 years.)
This is not in any way meant to take away from the efforts of any new engine devs or teams. I'm amazed that anyone is even trying given how large the platform has become and have great respect for those trying and succeeding.