It doesn't sound too unreasonable until you take several things into account:

1. The install base of thePS4+XBO is about as high as all those computers and consoles combined.
2. Marketing was much weaker and the game didn't have decades to build a following like Fifa.
3. Pirating games was widespread, so much that for many games only one in 10 copies were actually paid for on some computers.
4. Back in the day, selling over 100k copies was considered a smash hit. For instance, a computer smash hit like The Secret Of Monkey Island sold about 300k on a worldwide scale across all platforms.
5. Another comparison: WarCraft 2 managed to sell over 2M within 5 years (1993-1998), but again that was worldwide, not a single country alone. And that game got really hyped a lot back in the day and because CD-ROM drives that could burn CD's were very expensive at the time of release of the game, the amount of pirated copies for this game was very low before 1996.

Selling 2M in a single country was practically unheard of and the only instances at the time that managed this feat were Super Mario Bros and The Legend Of Zelda. Both were on consoles which were hard to bypass the copy protection on. But for Ghostbusters, most of the sales must have happened on computers with very low copy protection.

With all that in mind, I find the 2M claim really doubtful. It did sell well, and topped the charts in the UK for 3 months straight,  but 2M sold? Nope! Even if we were to add in the pirated copies, I find 2M to be a bit of a stretch.

Even re-released as part of a compilation 2m sounds a lot.